bridge.4: Document the net.link.bridge.log_mac_flap sysctl variable

It was introduced in 2d3614fb13 (bridge: Log MAC address port flapping).

Reviewed by:	gbe (manpages)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D39465

share/man/man4/bridge.4

@@ -35,7 +35,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 7, 2022
+.Dd April 10, 2023
 .Dt IF_BRIDGE 4
 .Os
 .Sh NAME
@@ -118,6 +118,15 @@ interfaces that are part of the bridge.
 For unicast traffic, the bridge learns which MAC addresses are associated
 with which interfaces and will forward the traffic selectively.
 .Pp
+By default the bridge logs MAC address port flapping to
+.Xr syslog 3 .
+This behavior can be disabled by setting the
+.Xr sysctl 8
+variable
+.Va net.link.bridge.log_mac_flap
+to
+.Li 0 .
+.Pp
 All the bridged member interfaces need to be up in order to pass network traffic.
 These can be enabled using
 .Xr ifconfig 8