[1.7.x] Fixed #14481 -- Documented implicit "through" class created by ManyToManyField.

Thanks to jonathanmorgan for the report and initial patch.

Backport of de90129070 from master

docs/ref/models/fields.txt

@@ -1349,6 +1349,19 @@ that control how the relationship functions.
     :ref:`extra data with a many-to-many relationship
     <intermediary-manytomany>`.
 
+    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:
+
+    * ``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.
+
+    This class can be used to query associated records for a given model
+    instance like a normal model.
+
 .. attribute:: ManyToManyField.through_fields
 
     .. versionadded:: 1.7