Minor revisions for NETWORK_LIVENESS spec

Hi Nick, just some small revisions for the NETWORK_LIVENESS spec, in my network_liveness branch...

https://gitweb.torproject.org/user/atagar/torspec.git/commit/?h=network_liveness&id=e0e81d078af8c747bf08309c1a0c013d2f98efc3

Shooting ya a patch rather than pushing this since I'm not sure if the version is right, and since you might have a reason for explicitly saying 'unrecognized material is ok'.