man8/bridge.8: document that "local" is default for "bridge fdb add"

man8/bridge.8: document that "local" is default for "bridge fdb add"

The bridge does this:

fdb_modify:
	/* Assume permanent */
	if (!(req.ndm.ndm_state&(NUD_PERMANENT|NUD_REACHABLE)))
		req.ndm.ndm_state |= NUD_PERMANENT;

So let's make the user aware of the fact that if they don't want local
entries, they need to specify some other flag like "static".

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