Box format doc improvements

These changes address most of the issues described in #3613.

However it's still unclear why use of the box catalog metadata JSON document is "highly recommended" given that:

* No explanation is given about how the catalog metadata is used if you create it as a local file.
* No explanation is given about how the catalog metadata could (or even should) be hosted online.
* If you create a box via vagrant cloud, it will be created automatically anyway.