libxl: correct some comments regarding event API and fds

libxl: correct some comments regarding event API and fds

* libxl may indeed register more than one callback for the same fd,
  with some restrictions.  The allowable range of responses to this by
  the application means that this should pose no problems for users.
  But the documentation comment should be fixed.

* Document the relaxed synchronicity semantics of the fd_modify
  registration callback.

* A couple of comments referred to old names for functions.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>