(#22180) Recapture class comments under Ruby 2.0 RDoc

RDoc 2+ has an RDoc::ClassModule.add_comment(comment,location) method
which is the preferred method of setting comments for a class.  RDoc
shipping with Ruby 1.9 seems to fall back to using the @comment, which
is how rdoc1 handled this, but Ruby 2.0's rdoc seems to be stricter and
requires use of add_comment.

This patch overrides the add_comment method in our PuppetModule,
PuppetClass, PuppetNode code objects with a version compatible with
either rdoc1 or 2.  It also tests for module level README comments being
incorporated.