lavu/pixfmt: Update the description for AV_PIX_FMT_QSV

Since D3D11 was introduced for QSV in FFmpeg 5.0, there is an implied
API/ABI change for user-supplied frames [1], hence update the
description for AV_PIX_FMT_QSV.

[1] https://ffmpeg.org/pipermail/ffmpeg-devel/2021-December/290444.html

Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>

libavutil/pixfmt.h

@@ -208,6 +208,34 @@ enum AVPixelFormat {
     /**
      * HW acceleration through QSV, data[3] contains a pointer to the
      * mfxFrameSurface1 structure.
+     *
+     * Before FFmpeg 5.0:
+     * mfxFrameSurface1.Data.MemId contains a pointer when importing
+     * the following frames as QSV frames:
+     *
+     * VAAPI:
+     * mfxFrameSurface1.Data.MemId contains a pointer to VASurfaceID
+     *
+     * DXVA2:
+     * mfxFrameSurface1.Data.MemId contains a pointer to IDirect3DSurface9
+     *
+     * FFmpeg 5.0 and above:
+     * mfxFrameSurface1.Data.MemId contains a pointer to the mfxHDLPair
+     * structure when importing the following frames as QSV frames:
+     *
+     * VAAPI:
+     * mfxHDLPair.first contains a VASurfaceID pointer.
+     * mfxHDLPair.second is always MFX_INFINITE.
+     *
+     * DXVA2:
+     * mfxHDLPair.first contains IDirect3DSurface9 pointer.
+     * mfxHDLPair.second is always MFX_INFINITE.
+     *
+     * D3D11:
+     * mfxHDLPair.first contains a ID3D11Texture2D pointer.
+     * mfxHDLPair.second contains the texture array index of the frame if the
+     * ID3D11Texture2D is an array texture, or always MFX_INFINITE if it is a
+     * normal texture.
      */
     AV_PIX_FMT_QSV,
     /**