Modified the implementation of get_object() to be consistent with the approach used elsewhere in the API. Also added documentation for get_object() which seems to have been accidentally omitted.

This is a BACKWARDS-INCOMPATIBLE CHANGE for anyone depending on the API for get_object() that was introduced (but not documented) in r14254.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14292 bcc190cf-cafb-0310-a4f2-bffc1f526a37