Allow `description` key on all schemas

[samuelcolvin]

We should allow an extra no-op description key on all schemas which can be anything and should be stored along-side the validator somehow, useful for reviewing schemas and understanding bugs.

We already do this in generate_self_schema.py.

[PrettyWood]

Probably a good first issue

[samuelcolvin]

For anyone coming to this and wanting to work on it, best to ask here about the best approach (and/or propose an approach) before getting started as there are a few different ways to go with this.