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

It was introduced in 2d3614fb132b (bridge: Log MAC address port flapping).

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

(cherry picked from commit 3fdb40d1befe7b561e7e5fa0b5c80aac41adc50d)