aa4489c71b key: Improve docs (Tobin C. Harding)
Pull request description:
`PublicKey` types are for verifying ECDSA signatures, when these docs where written there were no other types of signatures. With the addition of taproot these docs have become stale.
ACKs for top commit:
apoelstra:
ACK aa4489c71b
Tree-SHA512: bb24d82f2bf316f8907b1bf02132d454d21f0b13d57f06f09f9985bc7fbf7b36e6972a0fdaf3a68967577dbe1995f2a14fd06fddd38eb46718f04bca1c50a445