docs: update comments

hosts/idols/ai/impermanence.nix

@@ -15,7 +15,7 @@
   # There are two ways to clear the root filesystem on every boot:
   ##  1. use tmpfs for /
   ##  2. (btrfs/zfs only)take a blank snapshot of the root filesystem and revert to it on every boot via:
-  ##  3. boot.initrd.postDeviceCommands = ''
+  ##     boot.initrd.postDeviceCommands = ''
   ##       mkdir -p /run/mymount
   ##       mount -o subvol=/ /dev/disk/by-uuid/UUID /run/mymount
   ##       btrfs subvolume delete /run/mymount
@@ -23,6 +23,11 @@
   ##     '';
   #
   #  See also https://grahamc.com/blog/erase-your-darlings/
+
+
+  # NOTE: impermanence only mounts the directory/file list below to /persistent
+  # If the directory/file already exists in the root filesystem, you should
+  # move those files/directories to /persistent first!
   environment.persistence."/persistent" = {
     # sets the mount option x-gvfs-hide on all the bind mounts
     # to hide them from the file manager
@@ -46,6 +51,8 @@
     files = [
       "/etc/machine-id"
     ];
+
+    # the following directories will be passed to /persistent/home/$USER
     users.ryan = {
       directories = [
         "codes"