Document another wallet encryption concern

doc/security-warnings.md

@@ -29,6 +29,12 @@ Wallet encryption is disabled, for several reasons:
   your entire transaction graph (other than newly-detected spends, which suffer
   from the earlier issue).
 
+- We were concerned about the resistance of the algorithm used to derive wallet
+  encryption keys (inherited from Bitcoin) to dictionary attacks by a powerful
+  attacker. If and when we re-enable wallet encryption, it is likely to be with
+  a modern passphrase-based key derivation algorithm designed for greater
+  resistance to dictionary attack, such as Argon2i.
+
 You should use full-disk encryption (or encryption of your home directory) to
 protect your wallet at rest, and should assume (even unprivileged) users who are
 runnng on your OS can read your wallet.dat file.