doc: Fix preallocated memory grammar

The grammar on structs that implement `Context` using preallocated
memory is slightly incorrect. Attempt to improve the grammar.

Author: tcharding

src/context.rs

@@ -76,19 +76,19 @@ pub trait Signing: Context {}
 /// Marker trait for indicating that an instance of `Secp256k1` can be used for verification.
 pub trait Verification: Context {}
 
-/// Represents the set of capabilities needed for signing with a user preallocated memory.
+/// Represents the set of capabilities needed for signing (preallocated memory).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct SignOnlyPreallocated<'buf> {
     phantom: PhantomData<&'buf ()>,
 }
 
-/// Represents the set of capabilities needed for verification with a user preallocated memory.
+/// Represents the set of capabilities needed for verification (preallocated memory).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct VerifyOnlyPreallocated<'buf> {
     phantom: PhantomData<&'buf ()>,
 }
 
-/// Represents the set of all capabilities with a user preallocated memory.
+/// Represents the set of all capabilities (preallocated memory).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct AllPreallocated<'buf> {
     phantom: PhantomData<&'buf ()>,
@@ -301,7 +301,7 @@ unsafe impl<'buf> Context for AllPreallocated<'buf> {
 }
 
 impl<'buf, C: Context + 'buf> Secp256k1<C> {
-    /// Lets you create a context with preallocated buffer in a generic manner(sign/verify/all)
+    /// Lets you create a context with a preallocated buffer in a generic manner(sign/verify/all).
     pub fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Secp256k1<C>, Error> {
         #[cfg(target_arch = "wasm32")]
         ffi::types::sanity_checks_for_wasm();