Fixed #32983 -- Added system check for redundant related_name on symmetrical M2M fields.

Since ManyToManyFields defined with `symmetrical=True` do not add a
related field to the target model, including a `related_name` argument
will never do what the coder likely expects. This makes including
a related_name with a symmetrical model raise a system check warning.

ticket-32983

docs/ref/checks.txt

@@ -313,6 +313,8 @@ Related fields
   with a ``through`` model.
 * **fields.W344**: The field's intermediary table ``<table name>`` clashes with
   the table name of ``<model>``/``<model>.<field name>``.
+* **fields.W345**: ``related_name`` has no effect on ``ManyToManyField`` with a
+  symmetrical relationship, e.g. to "self".
 
 Models
 ------