docs: update hasher documentation (#6392)

Description
---
Updates the documentation for `DomainSeparatedBorshHasher`.

Motivation and Context
---
Some hashing is handled by `DomainSeparatedBorshHasher`, which uses
Borsh encoding under the hood instead of manually using length
prepending and byte slices. This PR updates the corresponding
documentation for clarity.

How Has This Been Tested?
---
No testing needed.

What process can a PR reviewer use to test or verify this change?
---
Confirm that the updated documentation is accurate.

hashing/src/borsh_hasher.rs

@@ -29,7 +29,10 @@ use borsh::{io, io::Write, BorshSerialize};
 use digest::Digest;
 use tari_crypto::hashing::DomainSeparation;
 
-/// Domain separated borsh-encoding hasher.
+/// A domain-separated hasher that uses Borsh internally to ensure hashing is canonical.
+///
+/// This assumes that any input type supports `BorshSerialize` canonically; that is, two different values of the same
+/// type must serialize distinctly.
 pub struct DomainSeparatedBorshHasher<M, D> {
     writer: WriteHashWrapper<D>,
     _m: PhantomData<M>,
@@ -50,6 +53,7 @@ impl<D: Digest + Default, M: DomainSeparation> DomainSeparatedBorshHasher<M, D>
         self.writer.0.finalize()
     }
 
+    /// Update the hasher using the Borsh encoding of the input, which is assumed to be canonical.
     pub fn update_consensus_encode<T: BorshSerialize>(&mut self, data: &T) {
         BorshSerialize::serialize(data, &mut self.writer)
             .expect("Incorrect implementation of BorshSerialize encountered. Implementations MUST be infallible.");
@@ -62,7 +66,9 @@ impl<D: Digest + Default, M: DomainSeparation> DomainSeparatedBorshHasher<M, D>
 }
 
 /// This private struct wraps a Digest and implements the Write trait to satisfy the consensus encoding trait.
-/// Do not use the DomainSeparatedHasher with this.
+///
+/// It's important not to use `DomainSeparatedHasher` with this, since that can inconsistently handle length prepending
+/// and render hashing inconsistent. It's fine to use it with `DomainSeparatedBorshHasher`.
 #[derive(Clone)]
 struct WriteHashWrapper<D>(D);