Add doc entry for the new GUC paramenter enable_group_by_reordering

0452b46 adds alternative orderings of group-by keys during the query
optimization. This new feature is controlled by the new GUC parameter
enable_group_by_reordering, which accidentally came without the documentation.
This commit adds the missing documentation for that GUC.

Reported-by: Bruce Momjian
Discussion: https://postgr.es/m/ZnDx2FYlba_OafQd%40momjian.us
Author: Andrei Lepikhov
Reviewed-by: Pavel Borisov, Alexander Korotkov

Author: akorotkov

doc/src/sgml/config.sgml

@@ -5341,6 +5341,25 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-enable-groupby-reordering" xreflabel="enable_group_by_reordering">
+      <term><varname>enable_group_by_reordering</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>enable_group_by_reordering</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Controls if the query planner will produce a plan which will provide
+        <literal>GROUP BY</literal> keys sorted in the order of keys of
+        a child node of the plan, such as an index scan.  When disabled, the
+        query planner will produce a plan with <literal>GROUP BY</literal>
+        keys only sorted to match the <literal>ORDER BY</literal> clause,
+        if any. When enabled, the planner will try to produce a more
+        efficient plan. The default value is <literal>on</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
       <term><varname>enable_hashagg</varname> (<type>boolean</type>)
       <indexterm>