Document that Dotfiles persistence feature doesn't work with empty directories

This bug was reported for Tails 0.22.1~rc1 but is also present on Tails 0.22.

See live-persist log in attachment for more details.

Feature Branch: doc/dotfiles

Original created by @sajolida on 6635 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information