Update documentation for multiple databases to reflect four default permissions

[Ryan Schave]

The last bullet point in ​https://docs.djangoproject.com/en/2.1/topics/db/multi-db/#behavior-of-contrib-apps indicates three permission objects are created for each model. I believe this is in reference to the default permissions that are created with each model. Django 2.1 introduced the view permission (​https://docs.djangoproject.com/en/dev/releases/2.1/#model-view-permission), bringing this number to four.

[Tim Graham]

I'll fix this and reference #29683. Closing as a duplicate.