Do trivial rustdoc improvements

bitcoin/src/crypto/ecdsa.rs

@@ -186,13 +186,13 @@ impl<'a> IntoIterator for &'a SerializedSignature {
 #[derive(Debug, Clone, PartialEq, Eq)]
 #[non_exhaustive]
 pub enum Error {
-    /// Hex decoding error
+    /// Hex decoding error.
     Hex(hex::HexToBytesError),
-    /// Non-standard sighash type
+    /// Non-standard sighash type.
     SighashType(NonStandardSighashTypeError),
-    /// Empty Signature
+    /// Signature was empty.
     EmptySignature,
-    /// A secp256k1 error
+    /// A secp256k1 error.
     Secp256k1(secp256k1::Error),
 }
 

bitcoin/src/crypto/key.rs

@@ -683,9 +683,9 @@ pub enum Error {
     Base58(base58::Error),
     /// A secp256k1 error.
     Secp256k1(secp256k1::Error),
-    /// Invalid key prefix error
+    /// Invalid key prefix error.
     InvalidKeyPrefix(u8),
-    /// Hex decoding error
+    /// Hex decoding error.
     Hex(hex::HexToArrayError),
     /// `PublicKey` hex should be 66 or 130 digits long.
     InvalidHexLength(usize),