Add documentaion for jsonpath syntax extensions

Author: Nikita Glukhov

doc/src/sgml/func.sgml

@@ -11753,6 +11753,14 @@ table2-mapping
 </programlisting>
     </para>
    </listitem>
+
+   <listitem>
+    <para>
+     Enclosing the path specification into square brackets
+     <literal>[]</literal> automatically wraps the path evaluation
+     result into an array.
+    </para>
+   </listitem>
   </itemizedlist>
 
    <sect3 id="strict-and-lax-modes">