Comment about ed25519 private key format on Sign (#2632)

Closes #2001

crypto/ed25519/ed25519.go

@@ -46,6 +46,12 @@ func (privKey PrivKeyEd25519) Bytes() []byte {
 }
 
 // Sign produces a signature on the provided message.
+// This assumes the privkey is wellformed in the golang format.
+// The first 32 bytes should be random,
+// corresponding to the normal ed25519 private key.
+// The latter 32 bytes should be the compressed public key.
+// If these conditions aren't met, Sign will panic or produce an
+// incorrect signature.
 func (privKey PrivKeyEd25519) Sign(msg []byte) ([]byte, error) {
 	signatureBytes := ed25519.Sign(privKey[:], msg)
 	return signatureBytes[:], nil