Documentation, even if this is MediaWiki
Sounds like a good idea.
Sounds nice, except for the part about using @since...do we really want that?
I don't see any reason not to have it, and a lot of reason to do have it. I figure it's not really possible to add it to all current code accuratly, but I'd be very nice if people just added it when writing new code.
I like almost all of your proposals, just a bit skeptical with the @since. I have noticed, you have been using this in your extensions but even if you tag something as @since 0.1 and in 0.2 the whole function changes and gets new arguments, I am not sure you would update it to @since 0.2 or add a note. One way would be using multiple @since in this case and adding a note behind that how it was different back then. Anyway, I have missed this many times in MW core, would be so much easier to keep backwards compatibility if this were documented properly! Especially for public functions but also for important constants and globals.
