Clarify $comment usage in vocabularies.

This ensures that comments can be used in structures such as
Hyper-Schema's LDOs without every specification needing to
define it separately.

Author: handrews

hyper-schema.json

@@ -49,6 +49,9 @@
                 "submissionSchema": {
                     "description": "Schema describing the data to submit along with the request",
                     "allOf": [ { "$ref": "#" } ]
+                },
+                "$comment": {
+                    "type": "string"
                 }
             }
         }

jsonschema-core.xml

@@ -583,6 +583,12 @@
                 editing this keyword.  The value of this keyword MAY be used in debug or error
                 output which is intended for developers making use of schemas.
 
+                Schema vocabularies SHOULD allow "$comment" within any object containing
+                vocabulary keywords.  Implementations MAY assume "$comment" is allowed
+                unless the vocabulary specifically forbids it.  Vocabularies MUST NOT
+                specify any effect of "$comment" beyond what is described in this
+                specification.
+
                 Tools that translate other media types or programming languages
                 to and from application/schema+json MAY choose convert that media type or
                 programming language's native comments to or from "$comment" values.