block: drbd: drbd_interval: Demote some kernel-doc abuses and fix another header

Fixes the following W=1 kernel build warning(s):

 drivers/block/drbd/drbd_interval.c:11: warning: Function parameter or member 'node' not described in 'interval_end'
 drivers/block/drbd/drbd_interval.c:26: warning: Function parameter or member 'root' not described in 'drbd_insert_interval'
 drivers/block/drbd/drbd_interval.c:26: warning: Function parameter or member 'this' not described in 'drbd_insert_interval'
 drivers/block/drbd/drbd_interval.c:70: warning: Function parameter or member 'root' not described in 'drbd_contains_interval'
 drivers/block/drbd/drbd_interval.c:96: warning: Function parameter or member 'root' not described in 'drbd_remove_interval'
 drivers/block/drbd/drbd_interval.c:96: warning: Function parameter or member 'this' not described in 'drbd_remove_interval'
 drivers/block/drbd/drbd_interval.c:113: warning: Function parameter or member 'root' not described in 'drbd_find_overlap'

Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: drbd-dev@lists.linbit.com
Cc: linux-block@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210312105530.2219008-3-lee.jones@linaro.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>