Document custom controls

As discussed on the list https://groups.google.com/d/msg/ol3-dev/LnN3IUNxtoM/K0glKxDU8nUJ
I'm just quoting what @fredj said on the list https://groups.google.com/d/msg/ol3-dev/LnN3IUNxtoM/nxXZ3_Nmh1EJ

I can remove that if it's not correct