BIP174: Specify PSBT_IN_PREVIOUS_TXID serialization order

This change updates descriptions for the PSBT_IN_PREVIOUS_TXID field to specify serialization order.

It was not straightforward the order with which to store the txid in a psbt. It's not clearly specified in the spec, and there are not many resources or parsers to aid in debugging PSBTv2. I found the BIP370 test vector "Case: PSBTv0 but with PSBT_IN_PREVIOUS_TXID." seems to imply that the order of this txid value in PSBT_IN_PREVIOUS_TXID should match network serialization order.

I'm torn on if "little endian" is the correct phrase to use here in conjunction with "txid". I usually think of a "txid" as the big endian, human-presented order of a txn hash, but maybe it's okay to use interchangeably.