uri: Add a description for UriComparisonMode (#5476)

Author: TimWolla

reference/uri/uri.uricomparisonmode.xml

@@ -7,6 +7,13 @@
   <section xml:id="enum.uri-uricomparisonmode.intro">
    &reftitle.intro;
    <simpara>
+    Specifies if the <literal>fragment</literal> component should affect
+    the result of a URI comparison.
+   </simpara>
+   <simpara>
+    If the fragment is excluded from the comparison, two URIs will be
+    compared as if neither of them has a <literal>fragment</literal>
+    component.
    </simpara>
   </section>