avutil/frame: Document avcodec_get_frame_class() and the option naming system

Missing docs found by: nevcairiel

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/frame.h

@@ -176,6 +176,10 @@ typedef struct AVFrameSideData {
  * Similarly fields that are marked as to be only accessed by
  * av_opt_ptr() can be reordered. This allows 2 forks to add fields
  * without breaking compatibility with each other.
+ *
+ * Fields can be accessed through AVOptions, the name string used, matches the
+ * C structure field name for fields accessable through AVOptions. The AVClass
+ * for AVFrame can be obtained from avcodec_get_frame_class()
  */
 typedef struct AVFrame {
 #define AV_NUM_DATA_POINTERS 8