[1.0.X] Minor update to documentation for many-to-many filter() calls.

Removed a potential ambiguity when describing how multiple conditions in one
filter() call are handled.

Backport of r9958 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9959 bcc190cf-cafb-0310-a4f2-bffc1f526a37