[1.0.X] Clarified documentation regarding Oracle's treatment of nulls and...

[1.0.X] Clarified documentation regarding Oracle's treatment of nulls and empty strings. Backport of [9976] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9977 bcc190cf-cafb-0310-a4f2-bffc1f526a37