add is supported flag to filters and add missing doc comment

src/UglyToad.PdfPig/Filters/Ascii85Filter.cs

@@ -5,6 +5,7 @@
     using System.IO;
     using Tokens;
 
+    /// <inheritdoc />
     /// <summary>
     /// ASCII 85 (Base85) is a binary to text encoding using 5 ASCII characters per 4 bytes of data.
     /// </summary>
@@ -24,6 +25,10 @@
             85 * 85 * 85 *85
         };
 
+        /// <inheritdoc />
+        public bool IsSupported { get; } = true;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             var asciiBuffer = new byte[5];

src/UglyToad.PdfPig/Filters/AsciiHexDecodeFilter.cs

@@ -5,6 +5,10 @@
     using System.IO;
     using Tokens;
 
+    /// <inheritdoc />
+    /// <summary>
+    /// Encodes/decodes data using the ASCII hexadecimal encoding where each byte is represented by two ASCII characters.
+    /// </summary>
     internal class AsciiHexDecodeFilter : IFilter
     {
         private static readonly short[] ReverseHex = 
@@ -22,6 +26,10 @@
             /* 100 */  13, 14, 15
         };
 
+        /// <inheritdoc />
+        public bool IsSupported { get; } = true;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             var pair = new byte[2];

src/UglyToad.PdfPig/Filters/CcittFaxDecodeFilter.cs

@@ -6,6 +6,10 @@
 
     internal class CcittFaxDecodeFilter : IFilter
     {
+        /// <inheritdoc />
+        public bool IsSupported { get; } = false;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             throw new NotSupportedException("The CCITT Fax Filter for image data is not currently supported. " +

src/UglyToad.PdfPig/Filters/DctDecodeFilter.cs

@@ -6,6 +6,10 @@
 
     internal class DctDecodeFilter : IFilter
     {
+        /// <inheritdoc />
+        public bool IsSupported { get; } = false;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             throw new NotSupportedException("The DST (Discrete Cosine Transform) Filter indicates data is encoded in JPEG format. " +

src/UglyToad.PdfPig/Filters/FlateFilter.cs

@@ -9,8 +9,12 @@
     using Tokens;
     using Util;
 
+    /// <inheritdoc />
     /// <summary>
-    /// 
+    /// The Flate filter is based on the public-domain zlib/deflate compression method, a variable-length Lempel-Ziv 
+    /// adaptive compression method cascaded with adaptive Huffman coding. 
+    /// It is fully defined in Internet RFCs 1950, ZLIB Compressed Data Format Specification, and
+    /// 1951, DEFLATE Compressed Data Format Specification
     /// </summary>
     /// <remarks>
     /// See section 3.3.3 of the spec (version 1.7) for details on the FlateDecode filter.
@@ -34,6 +38,10 @@
             this.log = log;
         }
 
+        /// <inheritdoc />
+        public bool IsSupported { get; } = true;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             if (input == null)

src/UglyToad.PdfPig/Filters/IFilter.cs

@@ -3,8 +3,24 @@
     using System.Collections.Generic;
     using Tokens;
 
+    /// <summary>
+    /// A filter is used in a PDF to encode/decode data either to compress it
+    /// or derive an ASCII representation of the data.
+    /// </summary>
     internal interface IFilter
     {
+        /// <summary>
+        /// Whether this library can decode information encoded using this filter.
+        /// </summary>
+        bool IsSupported { get; }
+
+        /// <summary>
+        /// Decodes data encoded using this filter type.
+        /// </summary>
+        /// <param name="input">The encoded bytes which were encoded using this filter.</param>
+        /// <param name="streamDictionary">The dictionary of the <see cref="StreamToken"/> (or other dictionary types, e.g. inline images) containing these bytes.</param>
+        /// <param name="filterIndex">The position of this filter in the pipeline used to encode data.</param>
+        /// <returns>The decoded bytes.</returns>
         byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex);
     }
 }

src/UglyToad.PdfPig/Filters/Jbig2DecodeFilter.cs

@@ -6,6 +6,10 @@
 
     internal class Jbig2DecodeFilter : IFilter
     {
+        /// <inheritdoc />
+        public bool IsSupported { get; } = false;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             throw new NotSupportedException("The JBIG2 Filter for monochrome image data is not currently supported. " +

src/UglyToad.PdfPig/Filters/JpxDecodeFilter.cs

@@ -6,6 +6,10 @@
 
     internal class JpxDecodeFilter : IFilter
     {
+        /// <inheritdoc />
+        public bool IsSupported { get; } = false;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             throw new NotSupportedException("The JPX Filter (JPEG2000) for image data is not currently supported. " +

src/UglyToad.PdfPig/Filters/LzwFilter.cs

@@ -5,6 +5,11 @@
     using Tokens;
     using Util;
 
+    /// <inheritdoc />
+    /// <summary>
+    /// The LZW (Lempel-Ziv-Welch) filter is a variable-length, adaptive compression method
+    /// that has been adopted as one of the standard compression methods in the Tag Image File Format (TIFF) standard. 
+    /// </summary>
     internal class LzwFilter : IFilter
     {
         private const int DefaultColors = 1;
@@ -27,6 +32,10 @@
             this.pngPredictor = pngPredictor ?? throw new ArgumentNullException(nameof(pngPredictor));
         }
 
+        /// <inheritdoc />
+        public bool IsSupported { get; } = true;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             var parameters = decodeParameterResolver.GetFilterParameters(streamDictionary, filterIndex);

src/UglyToad.PdfPig/Filters/RunLengthFilter.cs

@@ -4,10 +4,19 @@
     using System.IO;
     using Tokens;
 
+    /// <inheritdoc />
+    /// <summary>
+    /// The Run Length filterencodes data in a simple byte-oriented format based on run length.
+    /// The encoded data is a sequence of runs, where each run consists of a length byte followed by 1 to 128 bytes of data.
+    /// </summary>
     internal class RunLengthFilter : IFilter
     {
         private const byte EndOfDataLength = 128;
 
+        /// <inheritdoc />
+        public bool IsSupported { get; } = true;
+
+        /// <inheritdoc />
         public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
         {
             using (var memoryStream = new MemoryStream())

src/UglyToad.PdfPig/Geometry/PdfPath.cs

@@ -211,6 +211,10 @@ namespace UglyToad.PdfPig.Geometry
             commands.Add(new Close());
         }
 
+        /// <summary>
+        /// Gets a <see cref="PdfRectangle"/> which entirely contains the geometry of the defined path.
+        /// </summary>
+        /// <returns>For paths which don't define any geometry this returns <see langword="null"/>.</returns>
         public PdfRectangle? GetBoundingRectangle()
         {
             if (commands.Count == 0)