Improve documentation of adding icon to iOS application

The documentation being referred to:
https://doc.qt.io/qt-6/ios-platform-notes.html#icons
and especially the external link that we suggest user to refer to:
https://help.apple.com/xcode/mac/current/#/dev10510b1f7

As a first-time user who has to add an icon to my iOS app, I found some unclarities that affect my deployment:
1. The "asset catalogs" needs to be explicitly assigned to the target (the application target). Otherwise, the default "ALL_BUILD" won't work.
2. When actually add the icon, Apple doc says "choose the app icon set from the App Icons Source pop-up menu" - this is somehow not possible in my Xcode 15. There is no combo box to choose from. User has to type in the name of app icon, i.e. "AppIcon" by default.
3. The icon file cannot contain alpha channel.

We may want to clarify those in our doc.