modifications to main derivation sections

[markblundeberg]

This is a consolidation and rewrite of the main sections on prefix / derivation.

• Put emphasis on how recipients must behave, as this is where most complexity exists.
  Senders in turn need to respect this, and it is their responsibility to not generate
  funds-losing transactions.
• Introduce P2SH-multisig key sorting on outputs, to have the generated multisigs appear typical.
• Change how the outpoint is included in the secret derivation (bytestring concatenation instead of
  integer addition, which was incompletely described before).
• Clarify input-prefix computation in its own section; clarify target-prefix extraction from payment code.
• Carefully specify how wallets may classify & analyze input types by scanning the scriptSig alone.
  (this is what EC will do, for instance).