Merge rust-bitcoin/rust-secp256k1#805: musig: Add warning to SecretNonce::dangerous_into_bytes about encoding instability

8fd68a7a8b4e962c2a5139d0d4221c02e8a74412 musig: Add warning to `SecretNonce::dangerous_into_bytes` about encoding instability


Pull request description:

  Addresses #804 by documenting the potential issues. I think it's an ok-ish first pass, but I'm totally open to rewording it if anyone is inclined.


ACKs for top commit:
  tcharding:
    ACK 8fd68a7a8b4e962c2a5139d0d4221c02e8a74412
  apoelstra:
    ACK 8fd68a7a8b4e962c2a5139d0d4221c02e8a74412; successfully ran local tests


Tree-SHA512: 981463835f0935893f15fa691272631ec797a149020981ad983805868d54f677658fc51fd4cd1cd2e5d9ebf4e5dc8baccf6ddbbc12292b7ebbefe035e1579128

Author: chain-forgexcr45