avcodec/avcodec: more verbose documentation for time_base

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

libavcodec/avcodec.h

@@ -1346,6 +1346,8 @@ typedef struct AVCodecContext {
      * of which frame timestamps are represented. For fixed-fps content,
      * timebase should be 1/framerate and timestamp increments should be
      * identically 1.
+     * This often, but not always is the inverse of the frame rate or field rate
+     * for video.
      * - encoding: MUST be set by user.
      * - decoding: the use of this field for decoding is deprecated.
      *             Use framerate instead.