Improve docs for IOCTL_GNTDEV_MAP_GRANT_REF

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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 <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Juergen Gross <[email protected]>

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