Improve documentation of "cluster management" feature

MXS-1331 added new "cluster management" features to MaxScale 2.2. I see a bit of information in the documentation about this:

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale-22-maxctrl/#cluster

However, there isn't really much information at all. It even mentions a --hosts option, but doesn't really elaborate on how to provide this option or what format this option should be provided in.

We should probably add more details to the documentation about these new cluster management features in general.

Should we also add a tutorial on how to set up a "cluster" of MaxScale instances?:

https://mariadb.com/kb/en/mariadb-enterprise/maxscale-22-tutorials/

Should this "How to make MariaDB MaxScale High Available" tutorial be modified (or at least renamed) now that MaxScale has built-in "clustering" features?:

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale-22-how-to-make-mariadb-maxscale-high-available/