add docs for UIFeature.BulkUnverifiedSessionsReminder

[kerryarchibald]

With matrix-org/matrix-react-sdk#9657

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• add docs for UIFeature.BulkUnverifiedSessionsReminder (#23875). Contributed by @kerryarchibald.