Add documentation about db_constraint for FK and M2M in Multiple Databases section

[Timothée Mazzucotelli]

Working on a project that uses several databases, I have searched for a long time the best way to have "fake" or "cross-db" foreign keys between tables that are not on the same database.

I was very surprised when I recently heard about the db_constraint attribute in model fields ForeignKey and ManyToManyField, and wondered why I have found so many ugly hacks before this very Django feature.

Documentation in the Multiple Databases section was very useful, but it would have been a lot more if db_constraint was mentioned in it. And a link back to Multiple Databases from ForeignKey and ManyToManyField db_constraint in model fields reference would also be great.

In don't know when db_constraint appeared but this modification should of course apply to each concerned version of Django (I know 1.8 and following are).

​https://docs.djangoproject.com/en/1.9/topics/db/multi-db/
​https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.ForeignKey.db_constraint
​https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.ManyToManyField.db_constraint

[Timothée Mazzucotelli]

Actually db_constraint is not meant to allow relationships between tables that are not on the same database. Instead it just tells not to set database constraint. So I closed this ticket as invalid.