Document maktaba flag default values

Fixes #19.
Yes, intentional. Referencing a flag without a value actually doesn't define it, it just throws `ERROR(NotFound)` if it's missing. So if anyone was actually annotating a flag reference instead of the flag definition, that would be weird.

BTW, this is matched against the code after line continuations are joined, so we shouldn't have to worry about whether the comma is down on a continued line or anything.