Fixed #12505 -- Clarified the use of the extra argument in formsets. Thanks to...

Fixed #12505 -- Clarified the use of the extra argument in formsets. Thanks to Andreas for the report, and timo for the draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13218 bcc190cf-cafb-0310-a4f2-bffc1f526a37