docs: improve postResumeCommands subvolume handling in btrfs documentation

[dbeley]

Hello,

The current documentation for the postResumeCommands using btrfs subvolumes isn't working properly (creating old_roots directly inside btrfs_tmp will make the backup folders unreachable later on so the find won't find them even if the subvolumes are still there), I made the following updates:

• Added old_roots folder in the persistent directory to fix folder accessibility
• Modified find command with -mindepth 1 to exclude the parent folder

Also discussed in #258

Let me know what you think!