BIP174: Clarify proprietary fields are retained on finalization

[evanlinjin]

Note that the Input Finalizer retains PSBT_IN_PROPRIETARY fields it does not understand, treating them like unknown fields so it does not clear proprietary data it cannot interpret.

[evanlinjin]

This version has proprietary fields mentioned as a separate category than unknown fields.

The other option is:

All other data except the UTXO and unknown fields (including PSBT_IN_PROPRIETARY fields the Input Finalizer does not understand) in the input key-value map should be cleared from the PSBT. The UTXO should be kept to allow Transaction Extractors to verify the final network serialized transaction.

[achow101]

The other option is:

I prefer this

[evanlinjin]

@achow101 updated!

[achow101]

ACK a4668bb

[jonatack]

(Are the clarifications here and #2182 worth a changelog note?)