Currently in MediaWiki (but not when centralauth is installed), if you go to Special:Userlogout, it logs you out of your current session. It does not log you out of your other sessions.
CentralAuth on the other hand does log you out of all sessions (via changing the equivalent of user_token).
Its not 100% clear which is the most desirable behaviour. I personally lean towards the centralAuth behaviour. I think we should change vanilla mediawiki to be like CentralAuth.
If we want to keep vanilla mediawiki, at the very least there needs to be a button to log someone out of all sessions (Special:UserLogout/force ? Link from special:Preferences?)