Documentation/sysfs-block-rnbd: Add descriptions for remap_device and resize

Two sysfs entries, remap_device and resize, are missing.

Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Link: https://lore.kernel.org/r/20210419073722.15351-3-gi-oh.kim@ionos.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>