better document the prefix topic

[dbu]

http://symfony.com/doc/master/cmf/book/routing.html should explain the thing about PHPCR route document and prefix of repository path and the url path part.

see for example the confusion in http://stackoverflow.com/questions/32429804/how-to-create-a-route-with-custom-path-in-symfony-routingbundle-phpcr/32629797