docs(firebase_auth): Update federated-auth.md for Facebook limited login

[lachezartodorov]

Description

Currently the documentation shows an example that works only with the classic access tokens. In many situations on iOS, the Limited Facebook Login is used, which provides JWT instead of the classic token. To be able to handle it, Firebase Auth needs to include the raw value of the nonce, to be able to validate the JWT.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.