uri: Add description to UrlValidationErrorType (#5474)

Author: TimWolla

reference/uri/uri.whatwg.urlvalidationerrortype.xml

@@ -7,6 +7,7 @@
   <section xml:id="enum.uri-whatwg-urlvalidationerrortype.intro">
    &reftitle.intro;
    <simpara>
+    The possible validation errors defined in the <link xlink:href="&url.url.whatwg-url;">WHATWG URL Standard</link>.
    </simpara>
   </section>