doc/ffmpeg: Try to clarify that the input option -r is not the same as -framerate.

doc/ffmpeg.texi

@@ -473,6 +473,9 @@ Set frame rate (Hz value, fraction or abbreviation).
 
 As an input option, ignore any timestamps stored in the file and instead
 generate timestamps assuming constant frame rate @var{fps}.
+This is not the same as the @option{-framerate} option used for some input formats
+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
 frame rate @var{fps}.