zfs: add comments about secrets

zfs.nix

@@ -4,10 +4,13 @@
   ...
 }:
 let
+  # DO NOT CHANGE
+  # path is set via a zfs property
   zfs-key = "/etc/zfs-key";
 in
 {
   system.activationScripts = {
+    # TODO! replace with proper secrets management
     "zfs-key".text = ''
       #!/bin/sh
       rm -fr ${zfs-key} || true