docs(analytics): emphasize the need to install Async Storage

it was already documented, but not directly in analytics, and not mentioning
the consequence in analytics of not installing AsyncStorage

docs/analytics/usage/index.md

@@ -146,6 +146,12 @@ import analytics from '@react-native-firebase/analytics';
 const appInstanceId = await analytics().getAppInstanceId();
 ```
 
+### Web / Other platform instance id
+
+Ensure you have installed an Async Storage provider for Firebase to preserve the instance id. Failure to do so means the instance id will be reset every time the application terminates.
+
+The main documentation for "other platform" support contains [an example.](/platforms#async-storage)
+
 # Disable Ad Id usage on iOS
 
 Apple has a strict ban on the usage of Ad Ids ("IDFA") in Kids Category apps. They will not accept any app

docs/platforms.md

@@ -91,7 +91,7 @@ await firebase.initializeApp({ ... });
 
 If you do not provide an Async Storage implementation, we use an in memory implementation
 which will result in resetting the data every time your app is restarted, in the case of
-Firebase Auth this means your users have to sign in again.
+Firebase Auth this means your users have to sign in again and for Firebase Analytics the app will generate a new instance id and look like a new installation every time.
 
 ### Analytics