Added mandatory FirebaseOptions fo JavaDoc example. (#2778)

* Added mandatory FirebaseOptions fo JavaDoc example.

* added missing commas

* Addressed comments

Fixed platform of App ID example: "android" -> "web".
Added mandatory Firebase options to example of secondary FirebaseApp.

Author: andirayo

packages/firebase/index.d.ts

@@ -1117,6 +1117,8 @@ declare namespace firebase {
    * // https://console.firebase.google.com
    * firebase.initializeApp({
    *   apiKey: "AIza....",                             // Auth / General Use
+   *   applicationId: "1:27992087142:web:ce....",      // General Use
+   *   projectId: "my-firebase-project",               // General Use
    *   authDomain: "YOUR_APP.firebaseapp.com",         // Auth with popup/redirect
    *   databaseURL: "https://YOUR_APP.firebaseio.com", // Realtime Database
    *   storageBucket: "YOUR_APP.appspot.com",          // Storage
@@ -1129,9 +1131,12 @@ declare namespace firebase {
    *
    * // Initialize another app
    * var otherApp = firebase.initializeApp({
+   *   apiKey: "AIza....",
+   *   applicationId: "1:27992087142:web:ce....",
+   *   projectId: "my-firebase-project",
    *   databaseURL: "https://<OTHER_DATABASE_NAME>.firebaseio.com",
    *   storageBucket: "<OTHER_STORAGE_BUCKET>.appspot.com"
-   * }, "otherApp");
+   * }, "nameOfOtherApp");
    * ```
    *
    * @param options Options to configure the app's services.