mainEntityOfPage range includes URL.

This is important as it will allow a shorthand form in JSON-LD to be
supported by the definitions we publish in our JSON-LD context file.

"mainEntityOfPage": "",

vs

"mainEntityOfPage": { "@id": "" }

(we default to @id if a property expects URL as a value for rangeincludes)

data/schema.rdfa

@@ -7197,6 +7197,7 @@ Note that Event uses startDate/endDate instead of startTime/endTime, even when d
       </span>
       <span>Domain: <a property="http://schema.org/domainIncludes" href="http://schema.org/Thing">Thing</a></span>
       <span>Range: <a property="http://schema.org/rangeIncludes" href="http://schema.org/CreativeWork">CreativeWork</a></span>
+      <span>Range: <a property="http://schema.org/rangeIncludes" href="http://schema.org/URL">URL</a></span>
       <link property="http://schema.org/inverseOf" href="http://schema.org/mainEntity"/>
    </div>