title: Should I use `registerOnChange` or `valueChanges` on a form control
body: <p>It seems that I can listen to changes on the control using <code>registerOnChange</code> or <code>valueChanges</code> method. I'm confused as to when to use what. <code>valueChanges</code> returns observable, so it makes sense to work with it as it provides many handy methods. When to use <code>registerOnChange</code> then?</p>