video: improve GstVideoFormatUnpack docs

https://bugzilla.gnome.org/show_bug.cgi?id=754650

gst-libs/gst/video/video-format.h

@@ -253,9 +253,14 @@ typedef enum
  * @width: the amount of pixels to unpack.
  *
  * Unpacks @width pixels from the given planes and strides containing data of
- * format @info. The pixels will be unpacked into @dest which each component
- * interleaved. @dest should at least be big enough to hold @width *
- * n_components * size(unpack_format) bytes.
+ * format @info. The pixels will be unpacked into @dest with each component
+ * interleaved as per @info's unpack_format, which will usually be one of
+ * #GST_VIDEO_FORMAT_ARGB, #GST_VIDEO_FORMAT_AYUV, #GST_VIDEO_FORMAT_ARGB64 or
+ * #GST_VIDEO_FORMAT_AYUV64 depending on the format to unpack.
+ * @dest should at least be big enough to hold @width * bytes_per_pixel bytes
+ * where bytes_per_pixel relates to the unpack format and will usually be
+ * either 4 or 8 depending on the unpack format. bytes_per_pixel will be
+ * the same as the pixel stride for plane 0 for the above formats.
  *
  * For subsampled formats, the components will be duplicated in the destination
  * array. Reconstruction of the missing components can be performed in a