[iOS] Document the fact that UDP broadcasts require special entitlements

It is not intuitive/obvious that performing a broadcast requires https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_networking_multicast because:

• The entitlement name refers to "multicast", while we wanted to do a broadcast. Users might think that they don't need this simply because they don't call QUdpSocket::joinMulticastGroup()

• When running the "Broadcast Sender Example" on iOS, the user does get asked for networking permissions. This gives the impression that the required permissions have been granted, so it is not obvious why the broadcasted messages are not received by other devices.

• The "Broadcast Sender Example" works fine on the iOS Simulator without any special entitlements, but not on a real iOS device. This might lead the user to think that something is wrong with their real device.

See QTBUG-119331 for an example of confusion caused by the points above.

Therefore, it would be very helpful for us to mention this entitlement in our documentation. Good places to document this might be:

• https://doc.qt.io/qt-6/qudpsocket.html
• https://doc.qt.io/qt-6/qhostaddress.html#SpecialAddress-enum