Update docs/contenttypes.rst

Note additional arguments for ImageContent.
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
Merge branch 'master' into next

* master:
  Fix #386: Do not crash in feincms_nav if no extensions registered
  Update docs/contenttypes.rst

Conflicts:
	feincms/module/page/templatetags/feincms_page_tags.py