[docs] - Add example for custom token sign in for webApi

Author: KkevinLi

docs/AUTHENTICATION.md

@@ -461,6 +461,9 @@ Note that changing a password may fail if your login for this `email` was too lo
 ### Custom login
 Use this login type to authenticate against firebase using a token generated by your own backend server. See these [instructions on how to generate the authentication token](https://firebase.google.com/docs/auth/server).
 
+<details>
+ <summary>Native API</summary>
+
 ```js
   var token = "myBackendToken";
 
@@ -478,6 +481,17 @@ Use this login type to authenticate against firebase using a token generated by
       }
   );
 ```
+</details>
+
+<details>
+ <summary>Web API</summary>
+
+```typescript
+  firebaseWebApi.auth().signInWithCustomToken(token)
+      .then(result => console.log(JSON.stringify(result)))
+      .catch(error => console.log(JSON.stringify(error)));
+```
+</details>
 
 ### Facebook login