Improve documentation on reseting an app's migrations to zero

[Keryn Knight]

This came on IRC a while back, because someone didn't know how to effectively undo everything they'd done.

The entire documentation that I can find is as follows:
python manage.py migrate --help says Use the name "zero" to unapply all migrations, and so does ​this page. That's it.

It's not clear what unapplying migrations means, unless you're familiar with it already - it's not described ​on this page after searching for "unapply" or "undo" or "reset" or "restart" or "zero", nor is it clarified in the help text for the command, and the process of resetting an app's state to start-from-scratch isn't mentioned at all in the migrations docs, except for this tiny affordance.