Hide and document class_document_constants
Hide and document internal methods in the Ruby parser
Remove trailing whitespace
Fix parsing of attr_reader defined in a comment

After extracting parse_comment_attr from parse_comment attr_reader and
attr_writer broke as the type extracted from the directive was not used
in the method.

This broke due to insufficient tests in the parser.
Hide and document deduplicate_method_name
Document RDoc::Context#add