Document all known .flatpak-info keys

Some of the keys in the Instance group were missing.
Add them to the man page.

Closes: #2051
Approved by: alexlarsson
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