Clarify that the test suite is not a Style Guide

[Julian]

This seems to be a point of confusion, or even contention, so we need to document it.

The test suite is not a set of what schemas or instances one should write.

It's a verification of "specified" behavior.

We should clarify to end users that "just because you see it here, doesn't mean it's a good idea", and to implementors that "if it's here it's because that's the behavior prescribed by the spec regardless of whether it's a good idea or not to do it".

Style guides, and what users should or should not do, are separate things.