pixdesc: fix and extend doxy for av_pix_fmt_get_chroma_sub_sample()

The parameters in the docs are currently swapped.

libavutil/pixdesc.h

@@ -268,8 +268,8 @@ enum AVPixelFormat av_pix_fmt_desc_get_id(const AVPixFmtDescriptor *desc);
  * the pixel format AVPixFmtDescriptor.
  *
  * @param[in]  pix_fmt the pixel format
- * @param[out] h_shift store log2_chroma_h
- * @param[out] v_shift store log2_chroma_w
+ * @param[out] h_shift store log2_chroma_w (horizontal/width shift)
+ * @param[out] v_shift store log2_chroma_h (vertical/height shift)
  *
  * @return 0 on success, AVERROR(ENOSYS) on invalid or unknown pixel format
  */