Document priority for Doctrine event listeners and subscribers

[mpdude]

Doctrine event listeners and subscribers can be given a priority.

This should be documented at http://symfony.com/doc/2.8/doctrine/event_listeners_subscribers.html.

[javiereguiluz]

Thanks for reporting this issue. You are right ... but this was already reported in #7649 (and being fixed in #9944) so we must close this as a duplicate. Thanks!

[JarJak]

No, they can't. See doctrine/DoctrineBundle#157 (comment)