docs(auth): update account-exists guidance after fetchSignInMethodsForEmail removal

[Lyokone]

Description

FirebaseAuth.fetchSignInMethodsForEmail() was removed in firebase_auth 6.0.0 (#17562, for email-enumeration security reasons), but the error-code documentation still instructs users to call it.

Existing behavior — the docs for account-exists-with-different-credential and email-already-in-use tell the reader to call fetchSignInMethodsForEmail, sign in with one of the providers it returns, and then link the original credential. That first step is no longer possible from the public API, so the recovery flow as documented cannot be followed.

This PR rewrites that guidance to the flow that still works: ask the user to sign in with a provider already linked to the email address, then link the original credential with linkWithCredential.

Two smaller cleanups from the same removal:

• firebase_auth.dart had a stray // ignore: deprecated_member_use_from_same_package left inside the signInWithCredential doc comment. The deprecated member it suppressed is gone, and it had been dropped in mid-sentence, leaving Resolve this by asking the user to sign in using one of the returned providers with nothing returning them.
• AuthCredential.signInMethod described itself as "the sign-in method identifier returned in fetchSignInMethodsForEmail". That trailing clause is dropped.

Six doc sites across three files, all /// comments. No API, behavior, or signature changes.

Out of scope: firebase_auth_web/lib/src/interop/auth_interop.dart still references firebase.auth.Auth.fetchSignInMethodsForEmail, but that documents the underlying JS SDK method in the interop layer rather than the Dart API, so it is left alone.

Related Issues

Fixes #12909

Checklist

• 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).
  • N/A — documentation-only change, no behavior to test.
• All existing and new tests are passing.
  • Not run locally; nothing executable changed. CI to confirm.
• 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

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

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