Update ui.instantiateImageCodec docs to reflect what it does. (#13233)

Author: GaryQian

lib/ui/painting.dart

@@ -1653,10 +1653,9 @@ class Codec extends NativeFieldWrapperClass2 {
 ///
 /// The [targetWidth] and [targetHeight] arguments specify the size of the output
 /// image, in image pixels. If they are not equal to the intrinsic dimensions of the
-/// image, then the image will be scaled after being decoded. If exactly one of
-/// these two arguments is specified, then the aspect ratio will be maintained
-/// while forcing the image to match the specified dimension. If both are not
-/// specified, then the image maintains its real size.
+/// image, then the image will be scaled after being decoded. If only one dimension
+/// is specified, the omitted dimension will remain its original size. If both are
+/// not specified, then the image maintains its real size.
 ///
 /// The returned future can complete with an error if the image decoding has
 /// failed.