ublk: document auto buffer registration(UBLK_F_AUTO_BUF_REG)

Document recently merged feature auto buffer registration(UBLK_F_AUTO_BUF_REG).

Cc: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250611085632.109719-1-ming.lei@redhat.com


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