doc: reword and clarify default stream selection.

doc/ffmpeg.texi

@@ -61,10 +61,12 @@ The format option may be needed for raw input files.
 @chapter Stream selection
 @c man begin STREAM SELECTION
 
-By default ffmpeg tries to pick the "best" stream of each type present in input
-files and add them to each output file. For video, this means the highest
-resolution, for audio the highest channel count. For subtitle it's simply the
-first subtitle stream.
+By default ffmpeg includes only one stream of each type (video, audio, subtitle)
+present in the input files and adds them to each output file.  It picks the
+"best" of each based upon the following criteria; for video it is the stream
+with the highest resolution, for audio the stream with the most channels, for
+subtitle it's the first subtitle stream. In the case where several streams of
+the same type rate equally, the lowest numbered stream is chosen.
 
 You can disable some of those defaults by using @code{-vn/-an/-sn} options. For
 full manual control, use the @code{-map} option, which disables the defaults just