Improve the clarity of the BIP w.r.t. pubkeys in the following ways:
- be specific about the purpose of pubkey types in PSBT fields
("plain pubkey" alone doesn't say a lot, especially if it's used
repeatedly within a field)
- replace all uses of "plain pubkey" by "compressed pubkey"
(the only category that should matter is whether the pubkey type
is "x-only" or "plain")
- use consistent word order, e.g. prefer
"compressed aggregate public key" over "aggregate compressed public key"
- The aggregate pubkey in `PSBT_{IN,OUT}_MUSIG2_PARTICIPANT_PUBKEYS` does not have to
appear anywhere in the Taproot output.
- The plain pubkeys in `PSBT_IN_MUSIG2_PUB_NONCE` and
`PSBT_IN_MUSIG2_PARTIAL_SIG` must be either the output pubkey, or
appears in a script, and not the internal key.
