filters.texi: explain audio upsampling in loudnorm

Explain that audio will be upsampled to 192 kHz. Addresses issues mentioned in
issue 6570.
Signed-off-by: Werner Robitza <werner.robitza@gmail.com>

doc/filters.texi

@@ -3070,6 +3070,10 @@ EBU R128 loudness normalization. Includes both dynamic and linear normalization
 Support for both single pass (livestreams, files) and double pass (files) modes.
 This algorithm can target IL, LRA, and maximum true peak.
 
+To accurately detect true peaks, the audio stream will be upsampled to 192 kHz.
+Use the @code{-ar} option or @code{aresample} filter to set a different output
+sample rate.
+
 The filter accepts the following options:
 
 @table @option