macextras: clarify that it is not supported on iOS

Change-Id: Iaf40c7dbe6f2a7b380792e14c2a1a9f128026b04
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>