qemu-nbd: Enhance man page

qemu-nbd: Enhance man page

Document some useful qemu-nbd command lines. Mention some restrictions
on particular options, like -p being only for MBR images, or -c/-d
being Linux-only.  Update some text given the recent change to no
longer serve oldstyle protocol (missed in commit 7f7dfe2a).  Also,
consistently use trailing '.' in describing options.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20190117193658.16413-4-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>