docs(macos): make note that keysharing capability should be enabled (#13046)

Author: russellwheatley

docs/setup/_setup_prereq_ios.md

@@ -26,3 +26,5 @@
 If you don't already have a Flutter app, you can complete the [Get
 Started: Test Drive](//flutter.io/get-started/test-drive/#androidstudio) to
 create a new Flutter app using your preferred editor or IDE.
+
+Note: If you're targeting macOS or macOS Catalyst, you must add the [Keychain Sharing capability](https://firebase.google.com/docs/ios/troubleshooting-faq#macos-keychain-sharing) to your target. In Xcode, navigate to your target's *Signing & Capabilities* tab, and then click **+ Capabilities** to add a new capability.