Add documentation for RayCast2D about is_colliding and get_collider quirks

At first, I thought get_collider() wouldn't return anything if the object it was pointing to wasn't in contact with it, but it turns out this isn't the case (though I suppose this would be useful for cases like wind tunnels). So I added some documentation for get_collider() and is_collidiing() to help clarify this.

If it's a bug, I'll go open a ticket for that too.