doc/flatpak: Document FLATPAK_SYSTEM_CACHE_DIR

This environment variable has been used for a while; it's just
undocumented. Copy some information from a comment in flatpak-dir.c.

Closes: #2841
Approved by: matthiasclasen
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f