Merge pull request bitcoin#367 from jl2012/bip141commitment

BIP141: commitment clarification. BIP144: new diagram

Author: luke-jr

bip-0141.mediawiki

@@ -75,6 +75,8 @@ and the coinbase's input's witness must consist of a single 32-byte array for th
 
 If there are more than one scriptPubKey matching the pattern, the one with highest output index is assumed to be the commitment.
 
+If all transactions in a block do not have witness data, the commitment is optional.
+
 === Witness program ===
 
 A scriptPubKey (or redeemScript as defined in BIP16/P2SH) that consists of a 1-byte push opcode (for 0 to 16) followed by a data push between 2 and 32 bytes gets a new special meaning. The value of the first push is called the "version byte". The following byte vector pushed is called the "witness program".