[1.7.x] Fixed #24432 -- Added docs for ManyToManyFields that reside i…

…n and refer to the same model.

Backport of b9b8411 from master

docs/ref/models/fields.txt

@@ -1380,14 +1380,26 @@ that control how the relationship functions.
 
     If you don't specify an explicit ``through`` model, there is still an
     implicit ``through`` model class you can use to directly access the table
-    created to hold the association. It has three fields:
+    created to hold the association. It has three fields to link the models.
+
+    If the source and target models differ, the following fields are
+    generated:
 
     * ``id``: the primary key of the relation.
     * ``<containing_model>_id``: the ``id`` of the model that declares the
       ``ManyToManyField``.
     * ``<other_model>_id``: the ``id`` of the model that the
       ``ManyToManyField`` points to.
 
+    If the ``ManyToManyField`` points from and to the same model, the following 
+    fields are generated:
+
+    * ``id``: the primary key of the relation.
+    * ``from_<model>_id``: the ``id`` of the instance which points at the
+      model (i.e. the source instance).
+    * ``to_<model>_id``: the ``id`` of the instance to which the relationship
+      points (i.e. the target model instance).
+
     This class can be used to query associated records for a given model
     instance like a normal model.