Copy first pass stats documentation from AV1 to VP9
Copy first pass stats documentation from AV1 to VP9

Bug: webm:1707
Change-Id: Iae7eaa9ba681272b70b6dad17cd2247edab6ef79

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