Describe replay prevention for INTRODUCE2 cells in rend-spec.txt
We never documented Tor hidden services' replay-prevention behaviour for INTRODUCE2 cells in rend-spec.txt. We got the replay prevention wrong the first time, fixed a security bug in it without a Trac ticket, and changed it again to fix a major usability bug in #3460 (moved). Time to document that replay-prevention behaviour (and its history).