Update example configuration

The example configuration is functional but somewhat minimal. It also offers no help in figuring out which commonly used features should be enabled or not.

For servers, the proxy_protocol parameter should be mentioned as a way to reduce the amount of user account management that has to be done when the accounts do not use wildcards that cover both the client IP and the MaxScale IP.

For readwritesplit, a commented out section with transaction_replay and some of the parameters that can be enabled with it would be a way to improve the HA aspects of it. Another comment should explains how causal_reads solves most problems with single database applications that do writes with autocommit and then read the data that was just written.

The mariadbmon section is covered by MXS-4744.

The global configuration could have a section for some of the newer parameters like auto_tune that make the configuration of the cluster and its limits a simpler task.