I think the simplest way to document runtime config settings / environment variables in one single page that exhaustively lists all possible settings. I would put this in the Developer Quick Start guide.
Usually I see this done in a code block, with a short comment alongside each setting unless they are already clear:
// This is a list of all RocketChat config settings with default values:
// If you're using these in a docker compose file, prepend OVERWRITE_SETTING_ to each setting.
NOTIFICATIONS_SCHEDULE_DELAY_ONLINE: 1,
NOTIFICATIONS_SCHEDULE_DELAY_AWAY: 1,
NOTIFICATIONS_SCHEDULE_DELAY_OFFLINE: 1
... etc.
I presume such a config file already exists in some developer’s source code, right? Otherwise how do you remember and test all the settings?
If it does then you can basically just copy/paste it into a new docs page.