chore: fixed api docs typos and omissions (#3170)

Author: jonathanedey

etc/firebase-admin.api.md

@@ -132,6 +132,4 @@ export interface ServiceAccount {
     projectId?: string;
 }
 
-// (No @packageDocumentation comment for this package)
-
 ```

src/app/lifecycle.ts

@@ -253,7 +253,7 @@ export function initializeApp(options?: AppOptions, appName: string = DEFAULT_AP
 
 /**
  * Returns an existing {@link App} instance for the provided name. If no name
- * is provided the the default app name is used.
+ * is provided the default app name is used.
  *
  * @param appName - Optional name of the `App` instance.
  *

src/index.ts

@@ -35,6 +35,12 @@ https://firebase.google.com/docs/web/setup
   console.error(message);
 }
 
+/**
+ * Firebase App and SDK initialization.
+ *
+ * @packageDocumentation
+ */
+
 export { App, AppOptions, FirebaseArrayIndexError } from './app/core';
 export { initializeApp, getApp, getApps, deleteApp } from './app/lifecycle';