qemu-nbd: document -v behavior in respect to --fork in man

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230906093210.339585-8-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: Wording improvement]
Signed-off-by: Eric Blake <eblake@redhat.com>

docs/tools/qemu-nbd.rst

@@ -197,7 +197,9 @@ driver options if :option:`--image-opts` is specified.
 
 .. option:: -v, --verbose
 
-  Display extra debugging information.
+  Display extra debugging information. This option also keeps the original
+  *STDERR* stream open if the ``qemu-nbd`` process is daemonized due to
+  other options like :option:`--fork` or :option:`-c`.
 
 .. option:: -h, --help