docs: clarify that special config variables are not exposed to applications
docs: clarify that special config variables are not exposed to applications

Closes #4093

[ci skip]
claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,dokku/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,progrium/dokku,dokku/dokku