doc/filters: amix only supports float samples

Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -697,6 +697,11 @@ ffmpeg -i input.mkv -filter_complex "[0:1][0:2][0:3][0:4][0:5][0:6] amerge=input
 
 Mixes multiple audio inputs into a single output.
 
+Note that this filter only supports float samples (the @var{amerge}
+and @var{pan} audio filters support many formats). If the @var{amix}
+input has integer samples then @ref{aresample} will be automatically
+inserted to perform the conversion to float samples.
+
 For example
 @example
 ffmpeg -i INPUT1 -i INPUT2 -i INPUT3 -filter_complex amix=inputs=3:duration=first:dropout_transition=3 OUTPUT