Documentation: format of configuration files requires inferencing

[cfs-pure]

While not overly difficult to figure out after finding the default system-wide configuration file, the documentation on ClusterShell's configuration files does not actually describe the format of clush.conf. It does, however, describe groups.conf format several paragraphs after that section in the same document.

It is recommended to move the format description (which itself can be a one-liner that says "ConfigParser") up to the top of the page for instant context and clarity.