Khaled is unlikely to write any documentation for developers, as it would bound him to maintain a kind of backward compatibility. He's been using the "6000+id" formula for long, but you aren't getting any guarantees.
Don't wonder why, otherwise you'll probably receive a bunch of "mIRC's his baby" arguments. We are to accept the current state of things. My advice is to taylor your application to a specific version of mIRC. If things change, you'll have to update your application.
Of course it means you can't develop anything serious for mIRC.
Necroman, #mIRC @ Undernet