btrfs: Clarify why Traverse() is not used in PreviousLeaf()

PreviousLeaf() navigates by tree structure rather than key
value, making Traverse() unsuitable here. Add a comment to
explain this instead of leaving an open TODO question.

Change-Id: Icd93d483bea5d5dab939abbef1f9f628238041ed
Reviewed-on: https://review.haiku-os.org/c/haiku/+/10458
Reviewed-by: Adrien Destugues <[email protected]>
Haiku-Format: Haiku-format Bot <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/add-ons/kernel/file_systems/btrfs/BTree.cpp

@@ -760,7 +760,8 @@ BTree::RemoveEntries(Transaction& transaction, Path* path,
 status_t
 BTree::PreviousLeaf(Path* path) const
 {
-	// TODO: use Traverse() ???
+	// Traverse() is not used here because it searches by key value,
+	// while leaf navigation only requires following tree structure.
 	int level = 0;
 	int slot;
 	Node* node = NULL;