docs: clarify comment

Author: boray

src/lib/provable/merkle-tree-indexed.ts

@@ -104,7 +104,7 @@ class IndexedMerkleMapBase {
 
   /**
    * Public getter for the root that combines the the root of the indexed merkle tree and length.
-   * This ensures length is included in the commitment.
+   * This provides protection against a attack vector mentioned in https://github.com/o1-labs/o1js/pull/2114#issuecomment-2948339955
    */
   get root(): Field {
     return Poseidon.hash([this._internalRoot, this.length]);