I put the note about values being sent to all bindings in both classes because otherwise one looking at only `RACBinding` won't know sending values to other `RACBinding`s isn't the only way to forward values to a `RACBinding`. I'll turn it around though.
Also I explicitly mention subscribers because binding's aren't subjects, so sending a value to a binding does not imply sending it to it's subscribers. I'll add a note about that.