Add centos7 command to start config.service (#8229)

* Add centos7 command to start config.service

* Add some text around starting immediately

gpdb-doc/dita/admin_guide/workload_mgmt_resgroups.xml

@@ -647,10 +647,10 @@ ls -l &lt;cgroup_mount_point&gt;/memory/gpdb</codeblock>
             or <codeph>cgconfig</codeph> (Redhat/CentOS 6.x) at node start-up. For example,
             configure one of the following cgroup service commands in your preferred service
             auto-start tool: <ul>
-              <li> Redhat/CentOS 7.x and SuSE11+ systems:
-                <codeblock>sudo systemctl enable cgconfig.service</codeblock>
-              </li>
-              <li> Redhat/CentOS 6.x systems: <codeblock>sudo chkconfig cgconfig on</codeblock></li>
+              <li>Redhat/CentOS 7.x and SuSE11+ systems: <codeblock>sudo systemctl enable cgconfig.service</codeblock>
+                <p>To start the service immediately (without having to reboot)
+                  enter:<codeblock>sudo systemctl start cgconfig.service</codeblock></p></li>
+              <li>Redhat/CentOS 6.x systems: <codeblock>sudo chkconfig cgconfig on</codeblock></li>
             </ul>
             <p>You may choose a different method to recreate the Greenplum Database resource group
               cgroup hierarchies.</p>