Cwa-website: FAQ should inform that it's normal under iOS that background checks are shifted back daily for 2 hours

[Ein-Tim]

Feature description

The FAQ should inform that under iOS the background checks are postponed by ± 2 hours daily and that this, at the moment, is expected behaviour.

Problem and motivation

For _normal_ users it can be very confusing that the checks are not always made at the same time daily. See f.e.: this comment from somebody who's confused because of this and thinks the App is not working correctly

[tkowark]

Added with https://github.com/corona-warn-app/cwa-website/pull/310