Merge #453: Remove mentions of 32-byte slice from tweak APIs

e612458 Remove mentions of 32-byte slice from tweak APIs (Martin Habovštiak)

Pull request description:

  These methods accept `&Scalar`, not slice and `&Scalar` already guarantees 32-bytes, so this failure case is impossible.

ACKs for top commit:
  sanket1729:
    ACK e612458.
  apoelstra:
    ACK e612458

Tree-SHA512: f1c083756cb99b16b16764c4d603196a99b7bae864ca7f62908866667cf0218c459447b95298edc71de92c1abe1268a1c085495e1626bb0b9168e1af6aaf2af6

src/key.rs

@@ -270,8 +270,7 @@ impl SecretKey {
     ///
     /// # Errors
     ///
-    /// Returns an error if the resulting key would be invalid or if the tweak was not a 32-byte
-    /// length slice.
+    /// Returns an error if the resulting key would be invalid.
     #[inline]
     pub fn add_tweak(mut self, tweak: &Scalar) -> Result<SecretKey, Error> {
         unsafe {
@@ -301,8 +300,7 @@ impl SecretKey {
     ///
     /// # Errors
     ///
-    /// Returns an error if the resulting key would be invalid or if the tweak was not a 32-byte
-    /// length slice.
+    /// Returns an error if the resulting key would be invalid.
     #[inline]
     pub fn mul_tweak(mut self, tweak: &Scalar) -> Result<SecretKey, Error> {
         unsafe {
@@ -1255,7 +1253,7 @@ impl XOnlyPublicKey {
     ///
     /// # Errors
     ///
-    /// If the resulting key would be invalid or if the tweak was not a 32-byte length slice.
+    /// If the resulting key would be invalid.
     ///
     /// # Examples
     ///