Add memory side effects notes when using function pointers (#5174)

scripts/generate/templates/MerkleProof.js

@@ -26,6 +26,9 @@ import {Hashes} from "./Hashes.sol";
  * OpenZeppelin's JavaScript library generates Merkle trees that are safe
  * against this attack out of the box.
  *
+ * IMPORTANT: Consider memory side-effects when using custom hashing functions
+ * that access memory in an unsafe way.
+ *
  * NOTE: This library supports proof verification for merkle trees built using
  * custom _commutative_ hashing functions (i.e. \`H(a, b) == H(b, a)\`). Proving
  * leaf inclusion in trees built using non-commutative hashing functions requires