qmake: Document the "aux" template type.

Better late than never.

Change-Id: If3bbeb4dfe3a8d49ceb02d9c2d0f2eff71595105
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>