Add "reset" and "show" examples for "client_encoding" (#2811)

gpdb-doc/dita/admin_guide/load/topics/g-character-encoding.xml

@@ -29,6 +29,12 @@
       <p>The client-side character encoding can be changed for a session by setting the
       server configuration parameter <codeph>client_encoding</codeph></p>
       <codeblock>SET client_encoding TO 'latin1';
+</codeblock>
+      <p>Change the client-side character encoding back to the default value:</p>
+      <codeblock>RESET client_encoding;
+</codeblock>
+      <p>Show the current client-side character encoding setting:</p>
+      <codeblock>SHOW client_encoding;
 </codeblock>
     </body>
   </topic>