Fix leftover TODO.

Explain why schema-instance+json's "schema" parameter MAY include
schemas against which the instance may not validate.

Author: handrews

jsonschema-core.xml

@@ -872,7 +872,9 @@ User-Agent: product-name/5.4.1 so-cool-json-schema/1.0.2 curl/7.43.0
                                     a JSON Schema resource.  The instance SHOULD successfully
                                     validate against at least one of these schemas.
                                     Non-validating schemas MAY be included for purposes such
-                                    as [something about compatibility here?]
+                                    as allowing clients to make use of older versions of a schema
+                                    as long as the runtime instance validates against that
+                                    older version.
                                 </t>
                             </list>
                         </t>