Improve rustdocs for KeyPair

Currently the rustdocs for `KeyPair` are stale in regards to serde, we
_do_ implement `Serialize` and `Deserialize` for `KeyPair`.

Improve the rustdocs for `KeyPair` by removing stale docs and adding
docs on fixed width binary serialization.

src/key.rs

@@ -669,13 +669,10 @@ impl Ord for PublicKey {
 ///
 /// # Serde support
 ///
-/// [`Serialize`] and [`Deserialize`] are not implemented for this type, even with the `serde`
-/// feature active. This is due to security considerations, see the [`serde_keypair`] documentation
-/// for details.
-///
-/// If the `serde` and `global-context` features are active `KeyPair`s can be serialized and
-/// deserialized by annotating them with `#[serde(with = "secp256k1::serde_keypair")]`
-/// inside structs or enums for which [`Serialize`] and [`Deserialize`] are being derived.
+/// Implements de/serialization with the `serde` feature _and_ `global-context` feature enabled.
+/// Serializes the secret bytes _only_, this means serialized data is 32 bytes if the `bincode`
+/// crate is used for serialization, other binary formats may add metadata to the serialized data
+/// (e.g. cbor and tycho).
 ///
 /// # Examples
 ///