docs: firebase_auth description

[Jake-Mok-Nelson]

Description

Updating the firebase_auth description.

Firebase Auth now also supports web and will probably support more in the future. As a result the Android and IOS mention in the description is out of date.

Related Issues

Replace this paragraph with a list of issues related to this PR from the issue database. Indicate, which of these issues are resolved or fixed by this PR. Note that you'll have to prefix the issue numbers with flutter/flutter#.

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.

[Jake-Mok-Nelson]

I believe everything has passed except for one check (e2e / ios (packages/cloud_firestore/cloud_firestore/example) (pull_request)), @russellwheatley
I didn't change anything to do with that though so perhaps it was already failing. Will this be merged?