docs: expand docs around herokuish upgrades
docs: expand docs around herokuish upgrades

Refs https://github.com/dokku/dokku/pull/4063#discussion_r460257032

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