Improve docs for IOCTL_GNTDEV_MAP_GRANT_REF

--------------cKY3Ggs6VDUCSn4I6iN78sHA
Content-Type: multipart/mixed; boundary="------------g0T69ASidFiPhh4eOY4XzIg1"

--------------g0T69ASidFiPhh4eOY4XzIg1
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

The current implementation of gntdev guarantees that the first call to
IOCTL_GNTDEV_MAP_GRANT_REF will set @index to 0.  This is required to
use gntdev for Wayland, which is a future desire of Qubes OS.
Additionally, requesting zero grants results in an error, but this was
not documented either.  Document both of these.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/f66c5a4e-2034-00b5-a635-6983bd999c07@gmail.com


Signed-off-by: Juergen Gross <jgross@suse.com>