Allow to override BOM's groupId/artifactId

Closes gh-723

initializr-docs/src/main/asciidoc/configuration-guide.adoc

@@ -445,6 +445,8 @@ initializr:
         artifactId: acme-foo-dependencies
         mappings:
           - versionRange: "[1.2.3.RELEASE,1.3.0.RELEASE)"
+            groupId: com.example.bar
+            artifactId: acme-foo-bom
             version: Arcturus.SR6
           - versionRange: "[1.3.0.RELEASE,1.4.0.RELEASE)"
             version: Botein.SR7
@@ -461,7 +463,8 @@ initializr:
 The primary use case here is to map Spring Boot versions to the preferred or supported
 versions of the Foo project. You can also see that for the milestone and snapshot BOMs,
 additional repositories are declared because those artifacts are not in the default
-repository.
+repository. Initially the BOM was identified as `com.example.bar:acme-foo-bom` and renamed
+as of `Botein` to `com.example.foo:acme-foo-dependencies`.
 
 TIP: We also use the `x` trick in version ranges to avoid updating the range every time
 a new Spring Boot 1.5 bug fix release is available