Our costant for the P2WPKH satisfaction size was wrong: in
7ac87b8f99fc0897753ce57d48ea24adf495a633 we added 1 WU for the script
sig len - but actually, that's 4WU! This resulted in
P2WPKH_SATISFACTION_SIZE being equal to 109 instead of 112.
This also adds a comment for better readability.