Clarify m2m intermediate table naming strategy
|Reported by:||Ramiro Morales||Owned by:||nobody|
|Cc:||Triage Stage:||Ready for checkin|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
Reports #12614 and #12810 hint about the fact the "By default, this table name is generated using the names of the two tables being joined." phrase at http://docs.djangoproject.com/en/dev/ref/models/fields/#id1 is creating confusion among users. We should change that to state the table is named after the model field names.