schnorr: Improve rustdocs

Improve the rustdocs in the `schnorr` module by doing:

- Use third person tense
- Add full stops
- Use links and code ticks

src/schnorr.rs

@@ -73,7 +73,7 @@ impl str::FromStr for Signature {
 }
 
 impl Signature {
-    /// Creates a Signature directly from a slice
+    /// Creates a `Signature` directly from a slice.
     #[inline]
     pub fn from_slice(data: &[u8]) -> Result<Signature, Error> {
         match data.len() {
@@ -119,7 +119,7 @@ impl<C: Signing> Secp256k1<C> {
         }
     }
 
-    /// Create a schnorr signature internally using the ThreadRng random number
+    /// Creates a schnorr signature internally using the [`rand::rngs::ThreadRng`] random number
     /// generator to generate the auxiliary random data.
     #[cfg(feature = "rand-std")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rand-std")))]
@@ -127,12 +127,12 @@ impl<C: Signing> Secp256k1<C> {
         self.sign_schnorr_with_rng(msg, keypair, &mut rand::thread_rng())
     }
 
-    /// Create a schnorr signature without using any auxiliary random data.
+    /// Creates a schnorr signature without using any auxiliary random data.
     pub fn sign_schnorr_no_aux_rand(&self, msg: &Message, keypair: &KeyPair) -> Signature {
         self.sign_schnorr_helper(msg, keypair, ptr::null())
     }
 
-    /// Create a schnorr signature using the given auxiliary random data.
+    /// Creates a schnorr signature using the given auxiliary random data.
     pub fn sign_schnorr_with_aux_rand(
         &self,
         msg: &Message,
@@ -142,7 +142,7 @@ impl<C: Signing> Secp256k1<C> {
         self.sign_schnorr_helper(msg, keypair, aux_rand.as_c_ptr() as *const ffi::types::c_uchar)
     }
 
-    /// Create a schnorr signature using the given random number generator to
+    /// Creates a schnorr signature using the given random number generator to
     /// generate the auxiliary random data.
     #[cfg(feature = "rand")]
     #[cfg_attr(docsrs, doc(cfg(feature = "rand")))]
@@ -159,7 +159,7 @@ impl<C: Signing> Secp256k1<C> {
 }
 
 impl<C: Verification> Secp256k1<C> {
-    /// Verify a schnorr signature.
+    /// Verifies a schnorr signature.
     pub fn verify_schnorr(
         &self,
         sig: &Signature,