documentation: apps without models.py can register for db related signals

[Val Neekman]

Before Django 1.7, all apps in the INSTALLED_APPS required to have a models.py, and they could latch onto the database related signals such as the pre_syncdb and the post_syncdb. So the documentations didn't need to be specific.

However, with the introduction of the AppConfig, apps no longer required a models.py and hence the documentation could benefit from an extra line of explanation that ONLY the apps that have the models.py could register to the database related signals.

On this page:
​https://docs.djangoproject.com/en/dev/ref/applications/#django.apps.AppConfig.models_module

This:
"It may be None if the application doesn’t contain a models module."
Could be changed to:
"It may be None if the application doesn’t contain a models module. Please note that only applications with a models module can register for the database related signals such as pre_migrate / post_migrate signals.

[Val Neekman]

Correction:

This:
"It may be None if the application doesn’t contain a models module."
Could be changed to:
"It may be None if the application doesn’t contain a models module. Please note that the database related signals such as pre_migrate / post_migrate signals are only emitted for applications that have a models module.

[donjpacheco]

added line to django ref applications documentation

[Tim Graham <timograham@…>]

In 55fb16258888443bc70398248b6e681f3bc032c8:

Fixed #22028 -- Documented that database signals aren't emitted for apps that laack a models module.

Thanks un33k for the suggestion and donjpacheco for the draft patch.