AllCops deprecation message shows the config path

This was motivated by a project I'm working on that has a couple of `.rubocop.yml` configuration files. I periodically forget about the second of the two, and was initially confused by repeated deprecation warnings after upgrading and fixing what I thought was the only offense. In general, it seems like a nice gesture to provide users with the location of files which contain deprecated syntax.

I can squash in a Changelog update if this is deemed a good change.