Document custom controls

As discussed on the list https://groups.google.com/d/msg/ol3-dev/LnN3IUNxtoM/K0glKxDU8nUJ
again, I'm just quoting the list https://groups.google.com/d/msg/ol3-dev/LnN3IUNxtoM/1Y5jDQgcF7sJ

I assume this refers to fullscreencontrol, which I assume doesn't include elements outside the map viewport div