doc: add vf_crop keepaspect documentation

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/filters.texi

@@ -903,9 +903,14 @@ testing purposes.
 
 @section crop
 
-Crop the input video to @var{out_w}:@var{out_h}:@var{x}:@var{y}.
+Crop the input video to @var{out_w}:@var{out_h}:@var{x}:@var{y}:@var{keep_aspect}
 
-The parameters are expressions containing the following constants:
+The @var{keep_aspect} parameter is optional, if specified and set to a
+non-zero value will force the output display aspect ratio to be the
+same of the input, by changing the output sample aspect ratio.
+
+The @var{out_w}, @var{out_h}, @var{x}, @var{y} parameters are
+expressions containing the following constants:
 
 @table @option
 @item x, y