ed: update VerifyingKey::from_bytes with ZIP-215 info (#704)

Removes the previous warning that points are unvalidated: they're
validated using the ZIP-215 rules, which allows unreduced y-coordinates.
Points are ensured valid by performing decompression, which finds a
solution to the curve equation, or returns an error.

Adds references to ZIP-215 and #626
which is an issue about potentially adding support for the RFC8032/NIST
validation criteria in the future.

ed25519-dalek/src/verifying.rs

@@ -124,11 +124,8 @@ impl VerifyingKey {
 
  /// Construct a `VerifyingKey` from a slice of bytes.
  ///
- /// # Warning
- ///
- /// The caller is responsible for ensuring that the bytes passed into this
- /// method actually represent a `curve25519_dalek::curve::CompressedEdwardsY`
- /// and that said compressed point is actually a point on the curve.
+ /// Verifies the point is valid under [ZIP-215] rules. RFC 8032 / NIST point validation criteria
+ /// are currently unsupported (see [dalek-cryptography/curve25519-dalek#626]).
  ///
  /// # Example
  ///
@@ -156,6 +153,9 @@ impl VerifyingKey {
  ///
  /// A `Result` whose okay value is an EdDSA `VerifyingKey` or whose error value
  /// is a `SignatureError` describing the error that occurred.
+ ///
+ /// [ZIP-215]: https://zips.z.cash/zip-0215
+ /// [dalek-cryptography/curve25519-dalek#626]: https://github.com/dalek-cryptography/curve25519-dalek/issues/626
  #[inline]
  pub fn from_bytes(bytes: &[u8; PUBLIC_KEY_LENGTH]) -> Result<VerifyingKey, SignatureError> {
  let compressed = CompressedEdwardsY(*bytes);