BIP-390: allow musig() under rawtr()

musig() was previously specified as only usable under tr(), but the BIP-390 test vectors already include valid rawtr(musig(...)) descriptors and Bitcoin Core’s descriptor implementation accepts musig() as a generic KEY under both tr() and rawtr(). This change updates the wording to explicitly allow musig() inside rawtr(), aligning the specification with the existing test vectors and Core semantics without changing any behavior.

Specification