Noting maximum number of fingerprints/hashes that can be queried

In our tor-dev@ discussion Karsten mentioned that we could request at most 96
descriptors at a time when polling by their fingerprints...

https://lists.torproject.org/pipermail/tor-dev/2013-June/005005.html

This is a hardcoded limit in tor, so noting it in our spec...

https://gitweb.torproject.org/tor.git/blob/HEAD:/src/or/routerlist.c#l4435