[1.6.x] Fixed #18285 -- Documented that bulk_create doesn't work with…

… M2M relationships.

Backport of 9173d2c from master

docs/ref/models/querysets.txt

@@ -1523,6 +1523,7 @@ This has a number of caveats though:
 * It does not work with child models in a multi-table inheritance scenario.
 * If the model's primary key is an :class:`~django.db.models.AutoField` it
   does not retrieve and set the primary key attribute, as ``save()`` does.
+* It does not work with many-to-many relationships.
 
 The ``batch_size`` parameter controls how many objects are created in single
 query. The default is to create all objects in one batch, except for SQLite