docs: point to aresample from asyncts docs

"Fixes" Ticket2036

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

doc/filters.texi

@@ -414,6 +414,7 @@ Pass the audio source unchanged to the output.
 Pad the end of a audio stream with silence, this can be used together with
 -shortest to extend audio streams to the same length as the video stream.
 
+@anchor{aresample}
 @section aresample
 
 Resample the input audio to the specified parameters. If none are specified
@@ -770,6 +771,8 @@ ffmpeg -f lavfi -i amovie=silence.mp3,silencedetect=noise=0.0001 -f null -
 Synchronize audio data with timestamps by squeezing/stretching it and/or
 dropping samples/adding silence when needed.
 
+As an alternative, you can use @ref{aresample} to do squeezing/stretching.
+
 The filter accepts the following named parameters:
 @table @option