Clarify session_track_system_variables requirement for readwritesplit's causal_reads parameter

The documentation for the causal_reads parameter for readwritesplit has the following note:

Note: This feature requires MariaDB 10.2.16 or newer to function. In addition to this, the session_track_system_variables parameter must be set to last_gtid.

https://mariadb.com/kb/en/mariadb-maxscale-25-readwritesplit/#causal_reads

This seems to imply that you should do something like this:

SET GLOBAL session_track_system_variables='last_gtid';

and:

[mariadb]

session_track_system_variables=last_gtid

However, in 10.3 and later, this system variable has a non-empty default value that some users may want to keep: autocommit,character_set_client,character_set_connection,character_set_results,time_zone.

Therefore, it may be more appropriate for users to set it like this:

SET GLOBAL session_track_system_variables=CONCAT(@@global.session_track_system_variables, ',last_gtid');

and:

[mariadb]

session_track_system_variables=autocommit,character_set_client,character_set_connection,character_set_results,time_zone,last_gtid

Maybe it would be more appropriate for the documentation to say something like this?:

Note: This feature requires MariaDB 10.2.16 or newer to function. In addition to this, the session_track_system_variables parameter must include last_gtid in its list of tracked system variables.