add `%url-search-named-value` function

src/ui/pages/help.html

@@ -1032,6 +1032,10 @@
 					<li><code>%url-search-value&lt;index&gt;</code>: the value of the search parameter at the specified
 						index (e.g. "%url-search-value&lt;0&gt;" for the value of the first search parameter)
 					</li>
+					<li><code>%url-search-named-value&lt;name&gt;</code>: the value of the search parameter with the
+						specified name (e.g. "%url-search-named-value&lt;id&gt;" for the value of the search parameter
+						"id")
+					</li>
 					<li><code>%url-segment&lt;index&gt;</code>: the segment of the pathname at the specified index (e.g.
 						"%url-segment&lt;0&gt;" for the first segment of the pathname)
 					</li>