[1.2.X] Fixed #15492 -- Documented the `invalid_date` and `invalid_time` error...

[1.2.X] Fixed #15492 -- Documented the `invalid_date` and `invalid_time` error keys for !SplitDateTimeField. Thanks to lode for the report.

Backport of [15663] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15664 bcc190cf-cafb-0310-a4f2-bffc1f526a37