Minor readability improvements, typo fix

[Sjors]

• add reference to BIP66 in addition to BIP62
• state who picks ''e'' and ''s'' and does the revealing (less use of passive voice FTW)
• explicitly state option 3 was chosen
• remind reader where all the weird constants come from the sekp256k1 standard
• add missing subscript i

[jonasnick]

I'm not really convinced that BIP66 is relevant here because it doesn't seem to be related to the type of malleability discussed in the BIP, but only to malleability in the encoding. Otherwise ACK.