Update JSON Schema Draft to 2020-12 and make $ref resolution rules explicit (#2437)

* fix http link to json-schema.org

Signed-off-by: Mike Ralphson <[email protected]>

* fix http link to spec.commonmark.org

Signed-off-by: Mike Ralphson <[email protected]>

* Specify rules for $ref resolution

Signed-off-by: Mike Ralphson <[email protected]>

* Specify relative resolution rules for pathItem $ref and example externalValue

Signed-off-by: Mike Ralphson <[email protected]>

* Update JSON Schema draft links to 2020-12 IETF pages

Signed-off-by: Mike Ralphson <[email protected]>

* Make language about 'MUST be in the form of a ...' consistent

Signed-off-by: Mike Ralphson <[email protected]>

* Make it clear pathItem $refs don't need to be external now

Signed-off-by: Mike Ralphson <[email protected]>

* Make RFC links consistent with regard to spacing

Signed-off-by: Mike Ralphson <[email protected]>

* Allow a URI for example.externalValue fields

This makes it fall under the rules for relative references.

Signed-off-by: Mike Ralphson <[email protected]>

* Explicitly call out $ref as a Relative Reference

* Remove wording about what implementations SHOULD/MAY do with a $ref

* Prefer 'referenced document' to 'referrant document' for clarity

* Fix JSON Schema $ref resolution fallback rule

* Add links back to #relativeReferences definition

* Split #relativeReferences definition into URL and URI sections

Signed-off-by: Mike Ralphson <[email protected]>

Author: MikeRalphson