v9 docs for `getRedirectResult` need to be updated to reflect `null` return value

[fastfedora]

[REQUIRED] Describe your environment

• Operating System version: MacOS 12.5.1
• Browser version: Chromium 102.0.5005.63
• Firebase SDK version: 9.10.0
• Firebase Product: auth

[REQUIRED] Describe the problem

In v8, getRedirectResult returned a UserCredential with a null user if no redirect operation was called. In v9, it now returns null. While the method signature was changed for the v9 documentation, the description still says that a UserCredential will be returned with a null user, which is confusing. It was only after looking at the PR #5617, which made the actual change, that I could verify the new behavior.

This also should be added to the upgrade instructions for moving from v8 to v9, as this can break existing code if it is not changed.