qemu-nbd: Document --tls-creds

Commit 145614a1 introduced --tls-creds and documented it in
qemu-nbd.texi, but forgot to document it in 'qemu-nbd --help'.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20181003180426.602765-1-eblake@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>

qemu-nbd.c

@@ -94,6 +94,7 @@ static void usage(const char *name)
 "General purpose options:\n"
 "  --object type,id=ID,...   define an object such as 'secret' for providing\n"
 "                            passwords and/or encryption keys\n"
+"  --tls-creds=ID            use id of an earlier --object to provide TLS\n"
 "  -T, --trace [[enable=]<pattern>][,events=<file>][,file=<file>]\n"
 "                            specify tracing options\n"
 "  --fork                    fork off the server process and exit the parent\n"