Fix comment location marshalling in ClassModule

Similar to other code objects' marshalling behaviour, `ClassModule#comment_location`'s content
type would change before/after marshalling. But it should still be an array of pairs that respond
to the same set of methods.

This commit also adds documentation about the marshalling behaviour of `ClassModule#comment_location`.

Author: st0012

lib/rdoc/code_object/class_module.rb

@@ -40,6 +40,12 @@ class RDoc::ClassModule < RDoc::Context
   # After unmarshalling:
   # - +comment+ is an RDoc::Markup::Document
   # - +location+ is a filename String
+  #
+  # These type changes are acceptable (for now) because:
+  # - +comment+: Both String and Document respond to #empty?, and #parse
+  #   returns Document as-is (see RDoc::Text#parse)
+  # - +location+: Only used by #parse to set Document#file, which accepts
+  #   both TopLevel (extracts relative_name) and String
 
   attr_accessor :comment_location