docs(auth): add note about using user token vs uid (#5428)

* Added information about the ID Token which should be used to authenticate with the backend server.
* add getIdToken / uid to spellcheck dictionary

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

Author: ahron1

.spellcheck.dict.txt

@@ -53,6 +53,7 @@ firebase
 Firebase
 firebase-ios-sdk
 Firestore
+getIdToken
 GDPR
 globals
 Gradle
@@ -143,6 +144,7 @@ timezones
 triaging
 TypeDoc
 UI
+uid
 uncomment
 unhandled
 unsubscriber

docs/auth/usage/index.md

@@ -170,6 +170,10 @@ with the [`User`](/reference/auth/user) details.
 In case any errors occur, the module provides support for identifying what specifically went wrong by attaching a code
 to the error. For a full list of error codes available, view the [Firebase documentation](https://firebase.google.com/docs/reference/js/firebase.auth.Auth#error-codes_3).
 
+## Authenticate with backend server
+
+The user's token should be used for authentication with your backend systems. The token is fetched with the [getIdToken](https://rnfirebase.io/reference/auth/user#getIdToken) method. As mentioned in the [Firebase documentation](https://firebase.google.com/docs/auth/web/manage-users#get_a_users_profile), the uid should not be used for authentication.
+
 ## Signing out
 
 If you'd like to sign the user out of their current authentication state, call the `signOut` method: