xcb_take_socket: Document sequence wrap requirements

xcb_take_socket: Document sequence wrap requirements

If lots of requests are send without one causing a reply, xcb can get confused
about the current sequence number of a reply. Document the requirements of an
external socket owner to avoid this problem.

The return_socket callback for xcb_take_socket() originally was supposed to
return the last sequence number used, but the version committed to libxcb never
actually had this signature. This fixes the function's documentation not to
mention this non-existent return value.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Signed-off-by: Julien Danjou <julien@danjou.info>