Update docs/contenttypes.rst

Note additional arguments for ImageContent.
Revert commit 3ceb1a37d8927cb6e1e1768eb8452c9b55d950d4. The attempt at elegance caused a bug where reverse relations didn't always end up cached.
Merge branch 'patch-6' of git://github.com/bmihelac/feincms

* 'patch-6' of git://github.com/bmihelac/feincms:
  Update docs/contenttypes.rst
Fix #386: Do not crash in feincms_nav if no extensions registered