Added a point about Point&Size to the Rect dartdoc (#3405)

lib/ui/geometry.dart

@@ -481,6 +481,12 @@ class Point {
 
 /// An immutable 2D, axis-aligned, floating-point rectangle whose coordinates
 /// are relative to an origin point.
+///
+/// A Rect can be created with one its constructors or with a [Point] and
+/// a [Size] using the `&` operator:
+/// ```dart
+/// Rect myRect = const Point(1.0, 2.0) & const Size(3.0, 4.0);
+///```
 class Rect {
   Rect._();