avformat: Document where the AVOption names can be found for the fields of AVFormatContext

Missing docs found by: nevcairiel

RFC: should we add support so that the C field names always work as av option names/keys ?
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/avformat.h

@@ -1305,6 +1305,12 @@ typedef struct AVFormatInternal AVFormatInternal;
  * version bump.
  * sizeof(AVFormatContext) must not be used outside libav*, use
  * avformat_alloc_context() to create an AVFormatContext.
+ *
+ * Fields can be accessed through AVOptions (av_opt*),
+ * the name string used matches the associated command line parameter name and
+ * can be found in libavformat/options_table.h.
+ * The AVOption/command line parameter names differ in some cases from the C
+ * structure field names for historic reasons or brevity.
  */
 typedef struct AVFormatContext {
     /**