doc: add link for list-of-scalars mention

Reported-by: Manlio Perillo

Bug: 14016

Discussion: 20160311163928.6674.94707@wrigleys.postgresql.org

Reviewed-by: David G. Johnston

doc/src/sgml/func.sgml

@@ -14609,7 +14609,7 @@ WHERE EXISTS (SELECT 1 FROM tab2 WHERE col2 = tab1.col2);
    Boolean (true/false) results.
   </para>
 
-  <sect2>
+  <sect2 id="functions-comparisons-in-scalar">
    <title><literal>IN</literal></title>
 
 <synopsis>

doc/src/sgml/ref/values.sgml

@@ -219,7 +219,8 @@ WHERE ip_address IN (VALUES('192.168.0.1'::inet), ('192.168.0.10'), ('192.168.1.
   <tip>
    <para>
     For simple <literal>IN</> tests, it's better to rely on the
-    list-of-scalars form of <literal>IN</> than to write a <command>VALUES</>
+    <link linkend="functions-comparisons-in-scalar">list-of-scalars</link>
+    form of <literal>IN</> than to write a <command>VALUES</>
     query as shown above.  The list of scalars method requires less writing
     and is often more efficient.
    </para>