Document that "Securely clean available disk space" does not delete hidden files
Users cleaning available disk space* should be warn that sfill does not take hidden files in account, i.e previously delete files can still appear in trash folders.
This could be .trash folders from *nix, but also .DS_Store, ~/.Trash, /.Spotlight-V100 and others
* see the
Feature Branch: doc/7118-erase_available_disk_warning
- Related to #5831 (closed)