Document and optionally reduce risk of leaking secrets

[tarsius]

Please see the wip problem description at https://github.com/emacscollective/no-littering/blob/safer-backups/README.org#backup-files.

[tarsius]

Upstream issues:

• https://debbugs.gnu.org/cgi/bugreport.cgi?bug=62991
• https://github.com/emacsorphanage/undohist/issues/14
• https://codeberg.org/ideasman42/emacs-undo-fu-session/issues/6

[ideasman42]

I don't think this fix is correct. Although it boils down to emacs not having a good way to check if a file is a subdirectory of another (as far as I can see).

• temporary-file-directory may not have a trailing slash, in this case checking the prefix could falsely detect a temp directory e.g. /tmp_simple_example.
• The check may fail on case insensitive file systems (leaking details anyway).

This PR addresses the issues noted above https://codeberg.org/ideasman42/emacs-undo-fu-session/pulls/7