Clarify info about iterating over existing buckets

Co-authored-by: Benjamin Wang <[email protected]>
Signed-off-by: ErikKalkoken <[email protected]>

README.md

@@ -315,7 +315,7 @@ guarantee that they exist for future transactions.
 
 To delete a bucket, simply call the `Tx.DeleteBucket()` function.
 
-You can also iterate over all existing buckets with `Tx.forEach()`:
+You can also iterate over all existing top-level buckets with `Tx.forEach()`:
 
 ```go
 db.View(func(tx *bolt.Tx) error {