Describe key extraction correctly in rend-spec.txt
Right now, the key extraction algorithm in rend-spec.txt section 1.10 is incorrect. It implies that we only extract Kf and Kb, when we also extract Df and Db as we should. (see rend_client_receive_rendezvous() where it happens.)
We should fix the spec there.
Spotted by Frank Young at https://lists.torproject.org/pipermail/tor-dev/2013-April/004741.html