doc/filters: add metadata information for blackframe

Reviewed-by: Moritz Barsnick <barsnick@gmx.net>
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -4539,6 +4539,10 @@ 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.
 
+This filter exports frame metadata @code{lavfi.blackframe.pblack}.
+The value represents the percentage of pixels in the picture that
+are below the threshold value.
+
 It accepts the following parameters:
 
 @table @option