[1.1.X] Fixed #12601: Documented the slug field in the forms module. Backport...

[1.1.X] Fixed #12601: Documented the slug field in the forms module. Backport of [12673] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12674 bcc190cf-cafb-0310-a4f2-bffc1f526a37