doc/filters: update blackframe options description.

doc/filters.texi

@@ -1907,21 +1907,17 @@ the position in the file if known or -1 and the timestamp in seconds.
 In order to display the output lines, you need to set the loglevel at
 least to the AV_LOG_INFO value.
 
-The filter accepts parameters as a list of @var{key}=@var{value}
-pairs, separated by ":". If the key of the first options is omitted,
-the arguments are interpreted according to the syntax
-blackframe[=@var{amount}[:@var{threshold}]].
-
 The filter accepts the following options:
 
 @table @option
 
 @item amount
-The percentage of the pixels that have to be below the threshold, defaults to
+Set the percentage of the pixels that have to be below the threshold, defaults
-98.
+to @code{98}.
 
-@item threshold
+@item threshold, thresh
-Threshold below which a pixel value is considered black, defaults to 32.
+Set the threshold below which a pixel value is considered black, defaults to
+@code{32}.
 
 @end table