Merge rust-bitcoin#643: Update docs to mention ECDSA

aa4489c key: Improve docs (Tobin C. Harding)

Pull request description:

  `PublicKey` types are for verifying ECDSA signatures, when these docs where written there were no other types of signatures. With the addition of taproot these docs have become stale.

ACKs for top commit:
  apoelstra:
    ACK aa4489c

Tree-SHA512: bb24d82f2bf316f8907b1bf02132d454d21f0b13d57f06f09f9985bc7fbf7b36e6972a0fdaf3a68967577dbe1995f2a14fd06fddd38eb46718f04bca1c50a445

src/key.rs

@@ -21,7 +21,9 @@ use crate::{
 #[cfg(feature = "hashes")]
 use crate::{hashes, ThirtyTwoByteHash};
 
-/// Secret 256-bit key used as `x` in an ECDSA signature.
+/// Secret key - a 256-bit key used to create ECDSA and Taproot signatures.
+///
+/// This value should be generated using a [cryptographically secure pseudorandom number generator].
 ///
 /// # Side channel attacks
 ///
@@ -50,6 +52,7 @@ use crate::{hashes, ThirtyTwoByteHash};
 /// ```
 /// [`bincode`]: https://docs.rs/bincode
 /// [`cbor`]: https://docs.rs/cbor
+/// [cryptographically secure pseudorandom number generator]: https://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator
 #[derive(Copy, Clone)]
 pub struct SecretKey([u8; constants::SECRET_KEY_SIZE]);
 impl_display_secret!(SecretKey);
@@ -117,7 +120,7 @@ impl str::FromStr for SecretKey {
     }
 }
 
-/// A Secp256k1 public key, used for verification of signatures.
+/// Public key - used to verify ECDSA signatures and to do Taproot tweaks.
 ///
 /// # Serde support
 ///
@@ -1082,7 +1085,7 @@ impl CPtr for Keypair {
     fn as_mut_c_ptr(&mut self) -> *mut Self::Target { &mut self.0 }
 }
 
-/// An x-only public key, used for verification of schnorr signatures and serialized according to BIP-340.
+/// An x-only public key, used for verification of Taproot signatures and serialized according to BIP-340.
 ///
 /// # Serde support
 ///