Remove confusing sentence that implied the user should set the timebase.
Elaborate on how the timebase is set for muxing.
/**
* This is the fundamental unit of time (in seconds) in terms
- * of which frame timestamps are represented. For fixed-fps content,
- * time base should be 1/framerate and timestamp increments should be 1.
+ * of which frame timestamps are represented.
+ *
* decoding: set by libavformat
- * encoding: set by libavformat in av_write_header
+ * encoding: set by libavformat in av_write_header. The muxer may use the
+ * user-provided value of @ref AVCodecContext.time_base "codec->time_base"
+ * as a hint.
*/
AVRational time_base;