Fixed #21343 -- Noted many-to-many restrictions with intermediate model.

Thanks EvilDMP for the report.

docs/ref/models/relations.txt

@@ -138,10 +138,14 @@ Related objects reference
     .. note::
 
        Note that ``add()``, ``create()``, ``remove()``, and ``clear()`` all
-       apply database changes immediately for all types of related fields. In other
-       words, there is no need to call ``save()`` on either end of the
+       apply database changes immediately for all types of related fields. In
+       other words, there is no need to call ``save()`` on either end of the
        relationship.
 
+       Also, if you are using :ref:`an intermediate model
+       <intermediary-manytomany>` for a many-to-many relationship, some of the
+       related manager's methods are disabled.
+
 .. _direct-assignment:
 
 Direct Assignment

docs/topics/db/examples/many_to_many.txt

@@ -35,7 +35,10 @@ objects, and a ``Publication`` has multiple ``Article`` objects:
             ordering = ('headline',)
 
 What follows are examples of operations that can be performed using the Python
-API facilities.
+API facilities. Note that if you are using :ref:`an intermediate model
+<intermediary-manytomany>` for a many-to-many relationship, some of the related
+manager's methods are disabled, so some of these examples won't work with such
+models.
 
 Create a couple of ``Publications``::