docs: Clarify supported stanzas in app.json
docs: Clarify supported stanzas in app.json

Closes #2681

[ci skip]
jimjag/dokku,jimjag/dokku,progrium/dokku,alexquick/dokku,fruitl00p/dokku,alexquick/dokku,alexquick/dokku,claytonbrown/dokku,fruitl00p/dokku,claytonbrown/dokku,claytonbrown/dokku,fruitl00p/dokku,elia/dokku,elia/dokku,fruitl00p/dokku,dokku/dokku,progrium/dokku,elia/dokku,jimjag/dokku,claytonbrown/dokku,alexquick/dokku,elia/dokku,jimjag/dokku,jimjag/dokku,fruitl00p/dokku,claytonbrown/dokku,jimjag/dokku,fruitl00p/dokku,elia/dokku,progrium/dokku,progrium/dokku,fruitl00p/dokku,claytonbrown/dokku,dokku/dokku,jimjag/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,alexquick/dokku,elia/dokku,dokku/dokku,alexquick/dokku,progrium/dokku,jimjag/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,progrium/dokku,elia/dokku,dokku/dokku,progrium/dokku,fruitl00p/dokku,alexquick/dokku,dokku/dokku,elia/dokku,alexquick/dokku