Add clarity when `config` should be used in the `mediaType`

[SteveLasker]

We initially focused on putting config in the mediaType, and including a +json on the extension, simply as momentum for how they were represented at the time.
We later clarified config should be reserved when there's actually a config object, and the +json|+yaml should be reserved to represent how the optional config blob is formatted.

The current docs at Defining a Unique Artifact Type don't clarify config, and show examples of using config, even though no config object is actually used.

TODO: Add clarity to define an artifact type with a shortened string, only including config in the string if an actual config object is represented, including +json|+yaml|+txt to represent the actually config formatting.

[mikebrow]

need to consider if this is ready to close or reopen in the image spec

[mikebrow]

closing for now due to pending archive action.. pls reopen if archive is not completed and/or if you believe this close to be in error