document running_long_enough_to_decide_unreachable()

document running_long_enough_to_decide_unreachable()

It came as a surprise that Serge, the bridge authority, omits the Running
flag for all bridges in its first 30 minutes after a restart:
https://bugs.torproject.org/tpo/anti-censorship/rdsys/102

The fix we're doing for now is to accept it as correct behavior in
Tor, and change all the supporting tools to be able to handle bridge
networkstatus docs that have no Running bridges.

I'm documenting it here inside Tor too so the next person might not
be so surprised.