[1.8.x] Fixed #25082 -- Documented where to register system checks.

-------------------------------

Lastly, your check function must be registered explicitly with system check

registry.

registry. Checks should be registered in a file that's loaded when your

application is loaded; for example, in the :meth:`AppConfig.ready()

<django.apps.AppConfig.ready>` method.

.. function:: register(*tags)(function)