Document how to change log location

Note that some of this is already documented:

See: https://github.com/h2oai/sparkling-water/blob/master/doc/configuration/configuration_properties.rst

See: https://github.com/h2oai/sparkling-water/blob/master/doc/tutorials/change_log_level.rst

When the SW help system is completed, then this information might be easier for users to navigate.