Fixed #18907 -- Correct docs regard population of model backrefs.

But how is this possible, given that a model class doesn't know which other

model classes are related to it until those other model classes are loaded?

The answer lies in the :setting:`INSTALLED_APPS` setting. The first time any

model is loaded, Django iterates over every model in :setting:`INSTALLED_APPS`

and creates the backward relationships in memory as needed. Essentially, one of

the functions of :setting:`INSTALLED_APPS` is to tell Django the entire model

domain.

The answer lies in the :data:`app registry <django.apps.apps>`. When Django

starts, it imports each application listed in :setting:`INSTALLED_APPS`, and

then the ``models`` module inside each application. Whenever a new model class

is created, Django adds backward-relationships to any related models. If the

related models haven't been imported yet, Django keeps tracks of the

relationships and adds them when the related models eventually are imported.

For this reason, it's particularly important that all the models you're using

be defined in applications listed in :setting:`INSTALLED_APPS`. Otherwise,

backwards relations may not work properly.

Queries over related objects

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