Currently, the XRandR extension is enabled by default.

However, if you find the line below in Section "ServerLayout".

Option "RandR"     "False"

That means the XRandR extension is disabled. To enable it, you have to set this option to be "ture" or simply remove it.

Once the XRandR extension is enabled, the X config file should follow the configuration instructions demonstrated in part IV while the instructions in part III are no longer suitable to use.