doc/filters: extend syntax description for transpose, and add examples

doc/filters.texi

@@ -3946,7 +3946,12 @@ Default mode is @code{merge}.
 
 Transpose rows with columns in the input video and optionally flip it.
 
-This filter accepts the following named parameters:
+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 following syntax:
+@example
+transpose=@var{dir}:@var{passthrough}
+@end example
 
 @table @option
 @item dir
@@ -4005,6 +4010,17 @@ Preserve landscape geometry (when @var{width} >= @var{height}).
 Default value is @code{none}.
 @end table
 
+For example to rotate by 90 degrees clockwise and preserve portrait
+layout:
+@example
+transpose=dir=1:passthrough=portrait
+@end example
+
+The command above can also be specified as:
+@example
+transpose=1:portrait
+@end example
+
 @section unsharp
 
 Sharpen or blur the input video.