Fix API documentation issues

* 'Cookies' and 'Encodings' sections were not built because the
  reference to the functions was wrong.
* 'Exceptions' section had a wrong anchor link ('module-requests', same
   one as the main heading).
* Remove 'decode_gzip' function, which is no longer present.
Merge pull request #1337 from amercader/master

Fix minor issues on the API docs