avutil/frame: Add some very basic documentation for AVFrameSideData

Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/frame.h

@@ -124,6 +124,13 @@ enum AVActiveFormatDescription {
     AV_AFD_SP_4_3       = 15,
 };
 
+
+/**
+ * Structure to hold side data for an AVFrame.
+ *
+ * sizeof(AVFrameSideData) is not a part of the public ABI, so new fields may be added
+ * to the end with a minor bump.
+ */
 typedef struct AVFrameSideData {
     enum AVFrameSideDataType type;
     uint8_t *data;