There are four main routes of communication for discussing PHP documentation:
The main mailing list is the main avenue for chat and must be used when discussing major changes to the documentation. It's also used by people wanting to contribute to the project, and really anything related to the PHP documentation.
The doc-$LANG lists are dedicated to the translations. They include commit information but also are a place to discuss topics specific to a given translation.
IRC is often used when people are collaborating or brain storming for ideas, or for those times when a second set of eyes just seems like a good idea. Of course the mailing list works great for all of this too.
The Bugs database is used for, you guessed it, bugs.