Update option docs for when log config is used
Update option docs for when log config is used

When a logging configuration file is used (log_config_append is
specified), then most of the config options don't apply since all
the configuration is read from the config file rather than the
options.

Change-Id: I05a61acb221c62c22ea7830a63090657442e20be
Closes-Bug: 1370439

openstack/oslo.log