Update packaging tutorial and AppConfig docs to mandate setting default_auto_field for reusable apps

[Jacob Walls]

A reusable (third-party) app that:

• has models
• but does not set AppConfig.default_auto_field

is in a ​"completely broken" situation, given that any user attempting to use DEFAULT_AUTO_FIELD in their own settings will generate migrations for the reusable app, which can conflict with later migrations shipped by that app, or possibly get stored in the app's installation location:

Third-party apps should set default_auto_field so that the migrations the app includes match the models.

This discussion took place in a ​PR that was not accepted proposing to advise on user workarounds. Instead, I think we should advise app authors to do the right thing.

That would include:

• strengthening the wording in AppConfig.default_auto_field docs, replacing "You can use this... for third party applications" to something like "Third-party applications *must* set this ..."
• Fixing the tutorial to show this step under ​Packaging your app. To my knowledge (I'm hoping someone will confirm), when you follow the tutorial and arrive at this step modeling how to make your app reusable, there is no default_auto_field attribute on the AppConfig in apps.py.

This would be an approachable issue for a new contributor, as it involves following the tutorial to verify completeness and accuracy.

[Amar]

PR submitted: ​https://github.com/django/django/pull/20503

This patch:

• Updates the AppConfig.default_auto_field documentation to clarify that reusable applications must set this attribute
• Adds default_auto_field to the PollsConfig example in the packaging tutorial

This follows the guidance from the discussion in PR #20319.