Update src/memory_manager.rs

Co-authored-by: Islam El-Ashi <islam.elashi@dfinity.org>

Author: dragoljub-djuric

src/memory_manager.rs

@@ -297,6 +297,7 @@ struct MemoryManagerInner<M: Memory> {
     memory_buckets: BTreeMap<MemoryId, Vec<BucketId>>,
 
     // Tracks the buckets that were freed to be reused in future calls to `grow`.
+    // NOTE: A BTreeSet is used so that bucket IDs are maintained in sorted order.
     freed_buckets: BTreeSet<BucketId>,
 }