Doing so would mean the "models" module of the app that contains the
custom user model will be used in favour of the models module of the
wagtail.users app, which contains the UserProfile model. As a result,
the UserProfile model becomes mistakenly picked up by the wagtailcore
app, creating a bogus migration when makemigrations is run.
This partially reverts the docs to the version before
449a48d7f9, in particular the paragraphs
about creating the custom AppConfig subclass for wagtail.users.
Sage Abdullah