Fixed #26186 -- Documented how app relative relationships of abstract models behave.

This partially reverts commit bc7d201bdb.

Thanks Tim for the review.

Refs #25858.

docs/ref/models/fields.txt

@@ -1157,6 +1157,35 @@ you can use the name of the model, rather than the model object itself::
         # ...
         pass
 
+Relationships defined this way on :ref:`abstract models
+<abstract-base-classes>` are resolved when the model is subclassed as a
+concrete model and are not relative to the abstract model's ``app_label``:
+
+.. snippet::
+    :filename: products/models.py
+
+    from django.db import models
+
+    class AbstractCar(models.Model):
+        manufacturer = models.ForeignKey('Manufacturer', on_delete=models.CASCADE)
+
+        class Meta:
+            abstract = True
+
+.. snippet::
+    :filename: production/models.py
+
+    from django.db import models
+    from products.models import AbstractCar
+
+    class Manufacturer(models.Model):
+        pass
+
+    class Car(AbstractCar):
+        pass
+
+    # Car.manufacturer will point to `production.Manufacturer` here.
+
 To refer to models defined in another application, you can explicitly specify
 a model with the full application label. For example, if the ``Manufacturer``
 model above is defined in another application called ``production``, you'd