ios: explain how to define icons and launch images, smaller fixes

- how to add icons and launch images in the .pro, so that one
  does not need to do it in Xcode in a brittle way that will
  be lost when the xcode project is regenerated,
- clarify how to use assets catalogs,
- clarify that creator can be used in some places.

Task-number: QTBUG-42049
Change-Id: Ia8b9d95526c0de4098c5f06eb5817dc7734f216e
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>