Document the necessity to add on_delete param to old migrations

[karyon]

When going through the deprecation warnings i got when updating to django 1.9, i was surprised to find out that even after adding on_delete to all ForeignKeys i had and adding the according migration, the old migrations still threw those errors.

Apparently (see ​PR i'm supposed to edit old migrations. i think this is something that should go into the docs (release notes?), as i was pretty surprised by that and found the "solution" only after digging until i got to the PR.

[Luis San Pablo]

The documentation is already in the release notes:
​https://docs.djangoproject.com/en/1.9/releases/1.9/#foreignkey-and-onetoonefield-on-delete-argument
So I just updated the warning to explain that the migrations should also be upgraded.
​https://github.com/django/django/pull/5869

[Tim Graham <timograham@…>]

In a856555d:

Fixed #25981 -- Added need to update migrations to on_delete deprecation warning.

[Tim Graham <timograham@…>]

In 4f2eb5fc:

[1.9.x] Fixed #25981 -- Added need to update migrations to on_delete deprecation warning.

Backport of a856555df2a1b4ddad8bf21243de0bcbcc12fb20 from master