documentation: new best place to put the signal handler registration

[Val Neekman]

The new best place for registering the signal handlers are in the ready() of the AppConfig.

This way, the signals properly emit with the server starts and also via the django-admin shell.

The documentation would benefit from an update indicating the new feature of Django 1.7.

The following section may require the update.
​https://docs.djangoproject.com/en/dev/ref/signals/#django.db.models.signals.pre_migrate
​https://docs.djangoproject.com/en/dev/ref/signals/#django.db.models.signals.post_migrate

This:
"Any handlers that listen to this signal need to be written in a particular place: a management module in one of your INSTALLED_APPS. If handlers are registered anywhere else they may not be loaded by migrate."
Could be changed to:
"Any handlers that listen to this signal need to be written in a particular place: the AppConfig.ready() within the apps module in one of your INSTALLED_APPS. If handlers are registered anywhere else they may not be loaded by migrate."

[Val Neekman]

Also:

This:
"Sent by the migrate command before it starts to install an application."
Could be changed to:
"Sent by the migrate command before it starts to install an application that has a models module."

[Tim Graham]

I wonder if we need to mention this as something specific to these signals though? Can we assume all signals will now be registered in AppConfig.ready()?

[Aymeric Augustin]

I didn't check the commit logs, but the wording sounds like this is a warning against registering signals in urls.py (for instance). I don't think these signals are a special case any more.

[Val Neekman]

The current docs says that you "need to" register these signals in the management module or else.

This is not the case anymore ... AppConfig.ready() is a better place, right Aymeric?

Also, circular dependencies are no longer the issue if these signals are registering in AppConfig.ready() and yes, no need to register in urls.py are more.

In short: "need to" is no longer true.

[Tim Graham <timograham@…>]

In 5233b366935dcc81e47546921ab4eb5007c83719:

Fixed #22029 -- Removed obsolete advice on registering migrate/syncdb signal handlers.

All signals should now be registered in AppConfig.ready().

Thanks un33k for the report.

[Tim Graham <timograham@…>]

In 994274ea8315282484bc24aa403b2fd12b5bdeec:

[1.7.x] Fixed #22029 -- Removed obsolete advice on registering migrate/syncdb signal handlers.

All signals should now be registered in AppConfig.ready().

Thanks un33k for the report.

Backport of 5233b36693 from master