docs(messaging): add token management info to server integration doc (#5427)

Added additional section about handling tokens while logging out users, and about tokens being associated with the app installation instance. As discussed in the discussions #5423

Co-authored-by: Mike Hardy <[email protected]>

Author: ahron1

docs/messaging/server-integration.md

@@ -17,7 +17,7 @@ Firebase provides a number of SDKs in different languages such as [Node.JS](http
 [HTTP](https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages). These methods allow you to send messages
 directly to your user's devices via the FCM servers.
 
-# Device tokens
+## Device tokens
 
 To send a message to a device, you must access its unique token. A token is automatically generated by the device and
 can be accessed using the Cloud Messaging module. The token should be saved inside of your systems data-store and should
@@ -163,7 +163,20 @@ function App() {
 Your application code can then handle messages as you see fit; updating local cache, displaying a [notification](/messaging/notifications)
 or updating UI. The possibilities are endless!
 
-# Send messages to topics
+## Signing out users
+
+Firebase Cloud Messaging tokens are associated with the instance of the installed app. By default, only token expiration or uninstalling/reinstalling the app will generate a fresh token.
+
+This means that by default, if your app has users and you allow them to log out and log in on the same app on the same device, the same FCM token will be used for both users. Usually this is not what you want, so you must take care to cycle the FCM token at the same time you handle user logout/login.
+
+How and when you invalidate a token and generate a new one will be specific to your project, but a common pattern is to delete the FCM token during logout and update your back end to remove ti, then to fetch the FCM token during login and update your back end systems to associate the new token with the logged in user.
+
+<https://rnfirebase.io/reference/messaging#deleteToken>
+<https://rnfirebase.io/reference/messaging#getToken>
+
+Note that when a token is deleted by calling the `deleteToken` method, it is immediately and permanently invalid.
+
+## Send messages to topics
 
 When devices [subscribe to topics](/messaging/usage#topics), you can send messages without specifying/storing any device
 tokens.
@@ -198,8 +211,8 @@ To send a message to a combination of topics, specify a condition, which is a bo
 topics. For example, the following condition will send messages to devices that are subscribed to `weather` and either `news`
 or `traffic`:
 
-```
-'weather' in topics && ('news' in topics || 'traffic' in topics)
+```json
+condition: "'weather' in topics && ('news' in topics || 'traffic' in topics)"
 ```
 
 To send a message to this condition, replace the `topic` key with `condition`:
@@ -225,11 +238,11 @@ admin
   });
 ```
 
-# Send messages with image
+## Send messages with image
 
 Both the Notifications composer and the FCM API support image links in the message payload.
 
-## iOS
+### iOS
 
 To successfully send an image using the Admin SDK it's important that the `ApnsConfig` options are set:
 
@@ -254,7 +267,7 @@ const payload = {
 
 > Check out the [official Firebase documentation](https://firebase.google.com/docs/cloud-messaging/ios/send-image) to see the list of available configuration for iOS.
 
-## Android
+### Android
 
 Similarly to iOS, some configurations specific to Android are needed: