Document path argument for maxpasswd

$ maxpasswd --help

usage: maxpasswd [-h|--help] [path] password

This utility creates an encrypted password using a .secrets file

that has earlier been created using maxkeys.

-h, --help: Display this help.

path : The directory where the .secrets file is located, or the path

       of the .secrets file itself. Note that the name of the file

       must be .secrets.

If a path is not provided, the .secrets file is looked for in the

directory /var/lib/maxscale.