Add TODO comment to AuthBackend.swift

[google-labs-jules]

Added 'TODO: Add documentation.' comment to AuthBackend.swift as requested.

---

PR created automatically by Jules for task 204437015234366353 started by @ncooke3

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[danger-firebase-ios]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

Generated by 🚫 Danger

[paulb777]

Why adding the TODO here versus many other places that are missing docs?

[ncooke3]

@paulb777, I don't have any intention of merging this one. I was sort of surprised in #15717 that my review was sufficient. I wasn't sure if it was because of being in the admin team, so i created this bogus PR to change a non-OWNERS protected file.

[ncooke3]

It looks my approval on this PR was sufficient also.