doc: mention the noise_bsf parameter in the docs

This looks more like a seed than an amount, as the variable name
could lead to think.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/bitstream_filters.texi

@@ -21,7 +21,8 @@ name after a '='.
 ffmpeg -i INPUT -c:v copy -bsf:v filter1[=opt1=str1/opt2=str2][,filter2] OUTPUT
 @end example
 
-Below is a description of the currently available bitstream filters.
+Below is a description of the currently available bitstream filters,
+with their parameters, if any.
 
 @section aac_adtstoasc
 
@@ -143,9 +144,16 @@ ffmpeg -i frame_%d.jpg -c:v copy rotated.avi
 Damages the contents of packets without damaging the container. Can be
 used for fuzzing or testing error resilience/concealment.
 
+Parameters:
+A numeral string, whose value is related to how often output bytes will
+be modified. Therefore, values below or equal to 0 are forbidden, and
+the lower the more frequent bytes will be modified, with 1 meaning
+every byte is modified.
+
 @example
-ffmpeg -i INPUT -c copy -bsf noise output.mkv
+ffmpeg -i INPUT -c copy -bsf noise[=1] output.mkv
 @end example
+applies the modification to every byte.
 
 @section remove_extra