vpp: improve documentation for VAProcPipelineParameterBuffer.
vpp: improve documentation for VAProcPipelineParameterBuffer.

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