Fixed #15504 -- Cleaned up contrib.syndication and contrib.utils.feedgenerator docs. Corrected numerous reST problems, removed duplicate method declarations, corrected method signatures, etc. Thanks to slinkp for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15739 bcc190cf-cafb-0310-a4f2-bffc1f526a37