[4.2.x] Fixed #23528 -- Doc'd django.db.migrations.swappable_dependency().

Backport of 7d96ce68ae from main

docs/topics/migrations.txt

@@ -218,6 +218,26 @@ Apps without migrations must not have relations (``ForeignKey``,
 ``ManyToManyField``, etc.) to apps with migrations. Sometimes it may work, but
 it's not supported.
 
+Swappable dependencies
+----------------------
+
+.. function:: django.db.migrations.swappable_dependency(value)
+
+The ``swappable_dependency()`` function is used in migrations to declare
+"swappable" dependencies on migrations in the app of the swapped-in model.
+Currently, the first migration of this app and as a consequence the swapped-in
+model should be created in the initial migration. The argument ``value`` is a
+string ``"<app label>.<model>"`` describing an app label and a model name, e.g.
+``"myapp.MyModel"``.
+
+By using ``swappable_dependency()``, you inform the migration framework that
+the migration relies on a migration which sets up up a swappable model,
+allowing for the possibility of substituting the model with a different
+implementation in the future. This is typically used for referencing models
+that are subject to customization or replacement, such as the custom user
+model (``settings.AUTH_USER_MODEL``, which defaults to ``"auth.User"``) in
+Django's authentication system.
+
 .. _migration-files:
 
 Migration files