Document Rectangle constructor accepting a point + size

Rectangle constructors can accept a point and a size 

```lua
local pt = Point(2, 5)
local sz = Size(15, 7)
local rect = Rectangle(pt, sz)
-- Creates Rectangle(2, 5, 15, 7)
```

https://github.com/aseprite/aseprite/blob/main/src/app/script/rectangle_class.cpp