Update Bridging section for if_bridge(4), the deprecated bridge(4)
driver has been removed. The two last advanced bridging features
documented at the end of the page will be available on 6.3-R (in fact
one will be MFC next week and the other one with 6.3-R), but I chose
to not add "this feature is avaiblable on 6.3 blah blah" since they
are advanced features for experts and since they will be available on
RELENG_6 soon.

Submitted by: thompsa@