[Doc] Improve "Working with Product Observability" doc to state the order the RequestCorrelationIdValve should be added

[tharakawijekoon]

Description:
If the RequestCorrelationIdValve is added to the catalina-server.xml, it should come after the RequestEncodingValve to avoid encoding issues.

<Valve className="org.wso2.carbon.tomcat.ext.valves.RequestEncodingValve" encoding="UTF-8"/>

<Valve className="org.wso2.carbon.tomcat.ext.valves.RequestCorrelationIdValve"
                      headerToCorrelationIdMapping="{'customHeader':'Correlation-ID'}" queryToCorrelationIdMapping="{'RelayState':'Correlation-ID'}"/>

This should be documented.