uri: Improve explanation for UrlValidationError (#5472)

Author: TimWolla

reference/uri/uri.whatwg.urlvalidationerror.xml

@@ -7,6 +7,8 @@
   <section xml:id="uri-whatwg-urlvalidationerror.intro">
    &reftitle.intro;
    <simpara>
+    Provides details about errors that were detected when parsing a URL with
+    <classname>Uri\WhatWg\Url</classname>.
    </simpara>
   </section>
 
@@ -63,15 +65,16 @@
      <term><varname>type</varname></term>
      <listitem>
       <simpara>
-       A <classname>Uri\WhatWg\UrlValidationErrorType</classname> instance.
+       The type of error.
       </simpara>
      </listitem>
     </varlistentry>
     <varlistentry xml:id="uri-whatwg-urlvalidationerror.props.failure">
      <term><varname>failure</varname></term>
      <listitem>
       <simpara>
-       Whether the error refers to a failure, or a soft-error.
+       If &true; the error caused the URL to be rejected as invalid. If &false;
+       the error is a soft error that was automatically corrected during parsing.
       </simpara>
      </listitem>
     </varlistentry>