backlight: backlight: Improve backlight_device documentation

Improve the documentation for backlight_device and adapt it to
kernel-doc style.

The updated documentation is more strict on how locking is used.
With the update neither update_lock nor ops_lock may be used
outside the backlight core.

This restriction was introduced to keep the locking simple
by keeping it in the core.

It was verified that this documents the current state by renaming
update_lock => bl_update_lock and ops_lock => bl_ops_lock.
The rename did not reveal any uses outside the backlight core.
The rename is NOT part of this patch.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>