gbm: document that gbm_bo_map exposes a linear view

Drivers (Gallium, i965) expose a linear view of the buffer via
gbm_bo_map.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5238>