Change -bindTo: to not send unnecessary starting value.

Also detailed a bit better what the method does. It wasn't very clear compared to the `RACBind` documentation.

The current implementation works fine because even though the receiver's current value gets propagated, which is not supposed to happen, it is then overwritten by the other binding's value, so everything works as expected, but it falls apart when multiple schedulers are involved. This makes it a bit more correct and robust.

I haven't added any test for that because I don't want to give the impression the bindings are meant to work across multiple schedulers.