doc/filters: deflicker size option value is in frames

Signed-off-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Paul B Mahol <onemda@gmail.com> (in #ffmpeg-devel IRC)

doc/filters.texi

@@ -6261,7 +6261,7 @@ It accepts the following options:
 
 @table @option
 @item size, s
-Set moving-average filter size. Default is 5. Allowed range is 2 - 129.
+Set moving-average filter size in frames. Default is 5. Allowed range is 2 - 129.
 
 @item mode, m
 Set averaging mode to smooth temporal luminance variations.