[Xcb] documentation patches

[Michael Stapelberg]

Hi Peter,

Thanks for having a look.

Excerpts from Peter Harris's message of 2012-01-11 16:41:06 +0000:
> - I don't like needs_header. It's a non-example that is language
> specific. Also redundant, since the one and only use (xcb.h) is
> implicitly included by xproto.h already (at least for the language
> binding in question). In other cases, needs_header is implied by the
> <import> tag, and already handled by libxcb.
You are right, my mistake. I removed need_header.

> - "make check" fails on <doc> inside <reply>.
Also fixed, thanks.

Best regards,
Michael

BTW: The up-to-date version is on github. If you want me to re-send the patch,
just mention it.