docs(storage): document plaintext save caveat on WalletStore.Save

The password parameter accepts null, which saves the wallet in plaintext.
The XML doc comment now makes explicit that this is only acceptable when
the user has explicitly opted out of encryption with a UI warning shown.

src/Core/Storage/WalletStore.cs

@@ -25,6 +25,8 @@ public static class WalletStore
         return WalletDocument.FromJson(content);
     }
 
+    /// <param name="password">Null saves in plaintext. Only omit when the user has
+    /// explicitly opted out of encryption (UI must show a clear warning).</param>
     public static void Save(WalletDocument doc, string path, string? password = null)
     {
         var content = doc.ToJson();