docs - add superuser to gp_gpperfmon_send_interval GUC set classifications
<row>
<entry colname="col1">Any valid time expression (number and unit)</entry>
<entry colname="col2">1sec</entry>
- <entry colname="col3">master<p>system</p><p>restart</p></entry>
+ <entry colname="col3">master<p>system</p><p>restart</p><p>superuser</p></entry>
</row>
</tbody>
</tgroup>
