uapi nbd: improve doc links to userspace spec

The uapi <linux/nbd.h> header intentionally documents only the NBD
server features that the kernel module will utilize as a client.  But
while it already had one mention of skipped bits due to userspace
extensions, it did not actually direct the reader to the canonical
source to learn about those extensions.

While touching comments, fix an outdated reference that listed only
READ and WRITE as commands.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20230410180611.1051618-2-eblake@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>