vpp: explain filter flags more.
vpp: explain filter flags more.

chenxianqin/vaapi-libva,01org/iotg-lin-gfx-libva,01org/libva-cros,01org/iotg-lin-gfx-libva,pma1/libva,zzhhui/libva,chenxianqin/vaapi-libva,pma1/libva,chenxianqin/vaapi-libva,01org/libva-cros,gbeauchesne/libva,01org/libva-cros,zzhhui/libva,gbeauchesne/libva,zzhhui/libva,gbeauchesne/libva,01org/iotg-lin-gfx-libva,pma1/libva