BIP373: Clarify where keys in MuSig fields may appear in the Taproot output

- 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.

Specification