secp256k1: Generalize docs on Error

The `Error` type is the main general error used by this lib, it is not
specific to ECDSA. Fix the docs to show this.

src/lib.rs

@@ -299,7 +299,7 @@ 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