Update documentation: CRF works with VPX_Q mode
Update documentation: CRF works with VPX_Q mode

Tested that if VPX_Q is set, this variable will still be used
to pull the CRF value.

Change-Id: I065a219a7acd18b50478d4d0d3dc7ba5e1c90901

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