doc/ffmpeg: improve -r documentation

Explain different behavior for encoding and streamcopy.

doc/ffmpeg.texi

@@ -877,9 +877,20 @@ This is not the same as the @option{-framerate} option used for some input forma
 like image2 or v4l2 (it used to be the same in older versions of FFmpeg).
 If in doubt use @option{-framerate} instead of the input option @option{-r}.
 
-As an output option, duplicate or drop input frames to achieve constant output
+As an output option:
+@table @option
+@item video encoding
+Duplicate or drop frames right before encoding them to achieve constant output
 frame rate @var{fps}.
 
+@item video streamcopy
+Indicate to the muxer that @var{fps} is the stream frame rate. No data is
+dropped or duplicated in this case. This may produce invalid files if @var{fps}
+does not match the actual stream frame rate as determined by packet timestamps.
+See also the @code{setts} bitstream filter.
+
+@end table
+
 @item -fpsmax[:@var{stream_specifier}] @var{fps} (@emph{output,per-stream})
 Set maximum frame rate (Hz value, fraction or abbreviation).