minor #11770 Add tag name for MongoDB event listener (JonathanHolvey)

This PR was submitted for the 4.3 branch but it was merged into the 3.4 branch instead (closes #11770).

Discussion
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Add tag name for MongoDB event listener

I spent a couple of hours today trying to work out why my event listener wasn't working. It turns out that the tag name for the event listener service is different when using MongoDB, and it would be really useful to have this referenced in the documentation.

Commits
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25f375c Add tag name for MongoDB event listener

Author: javiereguiluz

doctrine/event_listeners_subscribers.rst

@@ -85,6 +85,11 @@ managers that use this connection.
             ->addTag('doctrine.event_subscriber', ['connection' => 'default'])
         ;
 
+.. tip::
+
+    If you're using Doctrine with MongoDB, you should use ``doctrine_mongodb.odm.event_listener``
+    as the tag name for the event listener service.
+
 Creating the Listener Class
 ---------------------------