Update protocol docs for "height".
Update protocol docs for "height".

Closes #269

Groestlcoin/electrumx-grs,erasmospunk/electrumx,shsmith/electrumx,thelazier/electrumx,erasmospunk/electrumx,Crowndev/electrumx,Groestlcoin/electrumx-grs,shsmith/electrumx,Crowndev/electrumx,thelazier/electrumx