Merge #530: secp256k1: Improve docs on Error

42bf99f Add full stop (Tobin C. Harding)
96f9fd3 Link to MESSAGE_SIZE in Error docs (Tobin C. Harding)
036bd0d secp256k1: Generalize docs on Error (Tobin C. Harding)

Pull request description:

  The docs on `Error` currently mention ECDSA which is no longer correct since we use this error in `schnorr` as well.

  Fix the `Error` doc issue described above and do a couple trivial other fixes while we are touching the code.

ACKs for top commit:
  apoelstra:
    ACK 42bf99f

Tree-SHA512: e1658d909c3588736570517a351e85c85c014ee31efe09fd262654dad873b0dcd3dbb6c8ad40866a24ee8e2e9bf0be99e5eb35251f6640222b953a0eb2a732ca

Author: apoelstra

src/lib.rs

@@ -299,13 +299,12 @@ impl fmt::Display for Message {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::LowerHex::fmt(self, f) }
 }
 
-/// An ECDSA error
+/// The main error type for this library.
 #[derive(Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Debug)]
 pub enum Error {
-    /// Signature failed verification
+    /// Signature failed verification.
     IncorrectSignature,
-    /// Badly sized message ("messages" are actually fixed-sized digests; see the `MESSAGE_SIZE`
-    /// constant).
+    /// Bad sized message ("messages" are actually fixed-sized digests [`constants::MESSAGE_SIZE`]).
     InvalidMessage,
     /// Bad public key.
     InvalidPublicKey,