[relax-ng] annotations

I think annotations are a necessity and that they should be distinctive. I
have tried to come up with alternatives to our current compact syntax, but I
just succeeded in convincing myself that what we have is fine.

Added a keyword for annotations, followed by braces...too verbose. Tried
different characters other than [, gained nothing.

The only thing I thought of was perhaps using /** and */ instead of [ and ],
but what do you use for embedded annotations?

I stand behind annotations as currently specified.

Mike