lavf/utils: document AVIndexEntry.timestamp

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avformat.h

@@ -469,7 +469,12 @@ enum AVStreamParseType {
 
 typedef struct AVIndexEntry {
     int64_t pos;
-    int64_t timestamp;
+    int64_t timestamp;        /**<
+                               * Timestamp in AVStream.time_base units, preferably the time from which on correctly decoded frames are available
+                               * when seeking to this entry. That means preferable PTS on keyframe based formats.
+                               * But demuxers can choose to store a different timestamp, if it is more convenient for the implementation or nothing better
+                               * is known
+                               */
 #define AVINDEX_KEYFRAME 0x0001
     int flags:2;
     int size:30; //Yeah, trying to keep the size of this small to reduce memory requirements (it is 24 vs. 32 bytes due to possible 8-byte alignment).