VACodedBufferSegment: modify the semantics/description
VACodedBufferSegment: modify the semantics/description

VACodedBufferSegment may contain more than one NALs, and flag
VA_CODED_BUF_STATUS_SINGLE_NALU will be used to singal the
segment only contain one NAL

Signed-off-by: Austin Yuan <f447c34e4ebcef84a81b40a8d4dbc448a14e5570@intel.com>

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