update the readme and document public properties

src/UglyToad.Pdf/Content/Letter.cs

@@ -2,6 +2,9 @@
 {
     using Geometry;
 
+    /// <summary>
+    /// A glyph or combination of glyphs (characters) drawn by a PDF content stream.
+    /// </summary>
     public class Letter
     {
         /// <summary>

src/UglyToad.Pdf/Content/Page.cs

@@ -20,8 +20,14 @@
 
         internal PageContent Content { get; }
 
+        /// <summary>
+        /// The set of <see cref="Letter"/>s drawn by the PDF content.
+        /// </summary>
         public IReadOnlyList<Letter> Letters => Content?.Letters ?? new Letter[0];
 
+        /// <summary>
+        /// The full text of all characters on the page in the order they are presented in the PDF content.
+        /// </summary>
         public string Text { get; }
 
         /// <summary>