Move django.utils.deprecation.django_file_prefixes to a dedicated warnings module.

[Jacob Walls]

I'm preparing a documentation section "Writing your own fetch mode" as suggested in a ​review of the feature.

Emitting a warning rather than an exception is the most obvious simple use case. I'd like my example to skip over Django frames when emitting the warning, just like we do for deprecations. So, I'd like to import and use django_file_prefixes() for this.

But:

• it's not documented (except for the contributing documentation, see 9932866e3a7693930a90a924014c10614820ea70)
• it lives in .deprecation

We have 4 +1s for moving it out of .deprecation in this ​review thread.

I propose to move it and document it (outside the contributor documentation), to make it public API.

[zky]

​https://github.com/django/django/pull/21423

[Jacob Walls <jacobtylerwalls@…>]

In f970a98e:

Fixed #37142 -- Moved django_file_prefixes() to django.utils.warnings.

[Natalia Bidart]

Jacob and I discussed these changes and we agreed the documentation additions should be reverted. While django_file_prefixes() is a handy utility that sets a good example for issuing warnings, it is an internal helper and documenting it implies public API support and a stability commitment we do not intend to make (at least not for now).

[nessita <124304+nessita@…>]

In b3af340:

Refs #37142 -- Removed docs for django.utils.warnings.django_file_prefixes().

This reverts docs changes from commit f970a98e464320c09dde8d45009eadfb4d038a57.