man8/bridge.8: document the "permanent" flag for "bridge fdb add"

man8/bridge.8: document the "permanent" flag for "bridge fdb add"

The bridge program parses "local" and "permanent" in just the same way,
so it makes sense to tell that to users:

fdb_modify:
		} else if (matches(*argv, "local") == 0 ||
			   matches(*argv, "permanent") == 0) {
			req.ndm.ndm_state |= NUD_PERMANENT;

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>