[1.0.X] Added a lot more explanation about form field validation, including

expanded examples. Fixed #5843, #6652, #7428.

Backport of r9177 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9180 bcc190cf-cafb-0310-a4f2-bffc1f526a37