Add XML documentation for Vector3/4's static vector properties, and their add/subtract methods.
Added XML docs for Vector2/3.Negate(). Holding off on Vector4 since I'm not sure if Vector4s are used in the same way (a direction) as Vector2/3s.
Merge pull request #2870 from dneelyep/patch-3

Add XML docs for Vector3/4.Add/Subtract/static vectors, and for Vector2/3.Negate