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>

man/man8/bridge.8

@@ -517,6 +517,10 @@ the interface to which this address is associated.
 - is a local permanent fdb entry
 .sp
 
+.B permanent
+- this is a synonym for "local"
+.sp
+
 .B static
 - is a static (no arp) fdb entry
 .sp