man/io_uring_register_buffers.3: mention why registered buffers are useful
man/io_uring_register_buffers.3: mention why registered buffers are useful

There are no details in this man page, so mention why it is in fact
beneficial to register buffers. This is specific to O_DIRECT file IO
for now, but the kernel side use cases will be expanded in the future.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing