Update doc string to reflect V9 changes

[dwyfrequency]

In v8, getRedirectResult returned a UserCredential with a null user if no redirect operation was called. In v9, it now returns null. Change docs to reflect this change

#6616

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Latest commit: 9ea8045

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (de32c24) and merge commit (a68d65c).

Test Logs

• Base (de32c24): https://github.com/firebase/firebase-js-sdk/actions/runs/3146138508
• Merge (a68d65c): https://github.com/firebase/firebase-js-sdk/actions/runs/3154367250

1. https://storage.googleapis.com/firebase-sdk-metric-reports/rrqWE6d8i5.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (de32c24) and merge commit (a68d65c).

Test Logs

• Base (de32c24): https://github.com/firebase/firebase-js-sdk/actions/runs/3146138508
• Merge (a68d65c): https://github.com/firebase/firebase-js-sdk/actions/runs/3154367250

1. https://storage.googleapis.com/firebase-sdk-metric-reports/9RBjwpEnru.html