Document qtmacextras functions

This patch aims to provide the missing Qt Mac Extras documentation

The conversion functions (e.g. NSData <=> QByteArray) are not
documented because they will either be moved to QtCore or
removed from QtMacExtras since already present in QtCore

Change-Id: I364be524966b353602bcf88218d9bb81042497ff
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>