Added a warning about nonexistent FK constraints when unmigrated apps depend on migrated ones.

Thanks NotSqrt for the report; refs #23741.

docs/topics/migrations.txt

@@ -213,8 +213,13 @@ will be.
 
 Be aware, however, that unmigrated apps cannot depend on migrated apps, by the
 very nature of not having migrations. This means that it is not generally
-possible to have an unmigrated app have a ForeignKey or ManyToManyField to
-a migrated app; some cases may work, but it will eventually fail.
+possible to have an unmigrated app have a ``ForeignKey`` or ``ManyToManyField``
+to a migrated app; some cases may work, but it will eventually fail.
+
+.. warning::
+
+    Even if things appear to work with unmigrated apps depending on migrated
+    apps, Django may not generate all the necessary foreign key constraints!
 
 This is particularly apparent if you use swappable models (e.g.
 ``AUTH_USER_MODEL``), as every app that uses swappable models will need