MXS-2257: Extend password encryption tutorial

Fixed the documentation on the arguments to maxkeys, which is a directory,
and added a short paragraph about alternative key file locations. Also
documented that keys are read from the directory where the `datadir`
parameter points to.

Documentation/Getting-Started/Configuration-Guide.md

@@ -468,6 +468,9 @@ Set the directory where the data files used by MariaDB MaxScale are stored.
 Modules can write to this directory and for example the binlogrouter uses this
 folder as the default location for storing binary logs.
 
+This is also the directory where the password encryption key is read from that
+is generated by `maxkeys`.
+
 ```
 datadir=/home/user/maxscale_data/
 ```