[1.9.x] Fixed #26186 -- Documented how app relative relationships of abstract models behave.

This partially reverts commit bc7d201bdbaeac14a49f51a9ef292d6312b4c45e.

Thanks Tim for the review.

Refs #25858.

Backport of 0223e213dd690b6b6e0669f836a20efb10998c83 from master

1 files changed, 29 insertions, 0 deletions

diff --git a/docs/ref/models/fields.txt b/docs/ref/models/fields.txt
index d8d047e3c9..20c50a0b12 100644
--- a/docs/ref/models/fields.txt
+++ b/docs/ref/models/fields.txt
@@ -1155,6 +1155,35 @@ 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