Update comments on `configure` call. (#5313)
Update comments on `configure` call. (#5313)

Currently it mentions that the calls are thread-safe. While technically true, `configure` should only be called from the main thread to ensure all SDKs are configured by the time they are used.
firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk