Clarify 'recognized' field in tor-spec

Our tor-spec left me pretty mystified what the 'recognized' field actually was. It discussed what to do when it was zero, but not what the field was or what non-zero meant. Thankfully Roger filled me in over tasty, tasty pizza.

Patch available in the 'recognized' branch of my repo...

https://gitweb.torproject.org/user/atagar/torspec.git/commit/?h=recognized