Document the node/kind fact.
Document the node/kind fact.

Summary:
Somehow we managed never to explicitly write down that the node/kind fact is
actually a required part of the schema for nodes. It is so fundamental that we
apparently took it on background. Let's fix that.

Reviewers: zarko, #core_team

Reviewed By: zarko, #core_team

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1875

benjyw/kythe,bzz/kythe,benjyw/kythe,benjyw/kythe,benjyw/kythe,bzz/kythe,benjyw/kythe,benjyw/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,benjyw/kythe,benjyw/kythe,kythe/kythe,kythe/kythe,bzz/kythe,bzz/kythe,bzz/kythe,benjyw/kythe,bzz/kythe,benjyw/kythe,kythe/kythe,kythe/kythe,benjyw/kythe,bzz/kythe,kythe/kythe,bzz/kythe,kythe/kythe,bzz/kythe,kythe/kythe,kythe/kythe,benjyw/kythe,bzz/kythe,kythe/kythe