Add rustdocs describing fixed width serde

We recently added fixed width serialization for some types however
serialization is only fixed width when data is serialized with the
`bincode` crate.

Add rustdocs describing fixed width serde to `SecretKey`, `PublicKey`,
and `XOnlyPublicKey` (`KeyPair` is already done).

src/key.rs

@@ -38,6 +38,12 @@ use schnorr;
 
 /// Secret 256-bit key used as `x` in an ECDSA signature.
 ///
+/// # Serde support
+///
+/// Implements de/serialization with the `serde` feature enabled. Serialized data is fixed width (32
+/// bytes) if you use `bincode` to do serialization, other binary formats may add metadata to the
+/// serialized data (e.g. cbor and tycho).
+///
 /// # Examples
 ///
 /// Basic usage:
@@ -73,6 +79,12 @@ pub const ONE_KEY: SecretKey = SecretKey([0, 0, 0, 0, 0, 0, 0, 0,
 
 /// A Secp256k1 public key, used for verification of signatures.
 ///
+/// # Serde support
+///
+/// Implements de/serialization with the `serde` feature enabled. Serialized data is fixed width (33
+/// bytes) if you use `bincode` to do serialization, other binary formats may add metadata to the
+/// serialized data (e.g. cbor and tycho).
+///
 /// # Examples
 ///
 /// Basic usage:
@@ -971,6 +983,12 @@ impl<'de> ::serde::Deserialize<'de> for KeyPair {
 
 /// An x-only public key, used for verification of Schnorr signatures and serialized according to BIP-340.
 ///
+/// # Serde support
+///
+/// Implements de/serialization with the `serde` feature enabled. Serialized data is fixed width (32
+/// bytes) if you use `bincode` to do serialization, other binary formats may add metadata to the
+/// serialized data (e.g. cbor and tycho).
+///
 /// # Examples
 ///
 /// Basic usage: