Document new options for supervisor backups

[mdegat01]

Proposed change

Document new option added in https://github.com/home-assistant/supervisor/pull/3814

Type of change

• [x] Document existing features within Home Assistant
• [ ] Document new or changing features which there is an existing pull request elsewhere
• [ ] Spelling or grammatical corrections, or rewording for improved clarity
• [ ] Changes to the backend of this documentation
• [ ] Removed stale or deprecated documentation

Additional information

• This PR fixes or closes issue: fixes #
• Link to relevant existing code or pull request: https://github.com/home-assistant/supervisor/pull/3814