Modcluster documentation enhancements

Document that the SELinux needs to be configured if present to run httpd + mod_cluster and provide example commands.

Mention that maybe you need to install the mod_cluster package separately from httpd, and where the mod_cluster.conf file is located.

Current docs: https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.4/html-single/configuration_guide/index#mod_cluster-config