docs: clarify permissions on persistent storage paths
docs: clarify permissions on persistent storage paths

CNB uses a custom user/group id depending on the image.

Also cleanup the entire doc to make it generally more readable with less repetition.

Closes #4479

dokku/dokku,progrium/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,dokku/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku