Document why we hash downloaded blocks for both sync algs (#2927)

## Proposed Changes
Initially the idea was to remove hashing of blocks in backfill sync. After considering it more, we conclude that we need to do it in both (forward and backfill) anyway. But since we forgot why we were doing it in the first place, this PR documents this logic. 

Future us should find it useful 


Co-authored-by: Divma <26765164+divagant-martian@users.noreply.github.com>

beacon_node/network/src/sync/manager.rs

@@ -131,7 +131,7 @@ pub enum SyncRequestType {
     RangeSync(Epoch, ChainId),
 }
 
-/// The result of processing a multiple blocks (a chain segment).
+/// The result of processing multiple blocks (a chain segment).
 #[derive(Debug)]
 pub enum BatchProcessResult {
     /// The batch was completed successfully. It carries whether the sent batch contained blocks.