Improve help message for arnr-type
Improve help message for arnr-type

BUG=webm:1346

Change-Id: Ia6c1cee3704a6b44515d883b4d0632ac567bc9a2

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx