Improve CallKit tutorial, that was missing important information.

ios/swift/4-CallKitTutorial/CallKitTutorial/CallKitProviderDelegate.swift

@@ -74,6 +74,11 @@ extension CallKitProviderDelegate: CXProviderDelegate {
 	
 	func provider(_ provider: CXProvider, perform action: CXAnswerCallAction) {
 		do {
+			// The audio stream is going to start shortly: the AVAudioSession must be configured now.
+			// It is worth to note that an application does not have permission to configure the
+			// AVAudioSession outside of this delegate action while it is running in background,
+			// which is usually the case in an incoming call scenario.
+			tutorialContext.mCore.configureAudioSession();
 			try tutorialContext.mCall?.accept()
 			tutorialContext.isCallRunning = true
 		} catch {
@@ -83,17 +88,27 @@ extension CallKitProviderDelegate: CXProviderDelegate {
 	}
 	
 	func provider(_ provider: CXProvider, perform action: CXSetHeldCallAction) {}
-	func provider(_ provider: CXProvider, perform action: CXStartCallAction) {}
+	func provider(_ provider: CXProvider, perform action: CXStartCallAction) {
+		// This tutorial is not doing outgoing calls. If it had to do so,
+		// configureAudioSession() shall be called from here, just before launching the
+		// call.
+		// tutorialContext.mCore.configureAudioSession();
+		// tutorialContext.mCore.invite("sip:bob@example.net");
+		// action.fulfill();
+	}
 	func provider(_ provider: CXProvider, perform action: CXSetMutedCallAction) {}
 	func provider(_ provider: CXProvider, perform action: CXPlayDTMFCallAction) {}
 	func provider(_ provider: CXProvider, timedOutPerforming action: CXAction) {}
 	func providerDidReset(_ provider: CXProvider) {}
 	
 	func provider(_ provider: CXProvider, didActivate audioSession: AVAudioSession) {
+		// The linphone Core must be notified that CallKit has activated the AVAudioSession
+		// in order to start streaming audio.
 		tutorialContext.mCore.activateAudioSession(actived: true)
 	}
 	
 	func provider(_ provider: CXProvider, didDeactivate audioSession: AVAudioSession) {
+		// The linphone Core must be notified that CallKit has deactivated the AVAudioSession.
 		tutorialContext.mCore.activateAudioSession(actived: false)
 	}
 }