Minor documentation additions

Added docs for Vector2.Zero/One/UnitX/UnitY, Rectangle.ToString(), and fixed an odd-sounding sentence. Also added docs for Vector2.Add/Subtract.
Ok, modified this to state:
"Gets or sets a bool that enables usage of Alt+F4 for window closing on desktop platforms. Value is true by default."
This seems the most simple and clear - can modify it as well if desired.