Add SYSTEM_EXEMPT_FROM_DISMISSIBLE_NOTIFICATIONS mention for Android 14 persistent notification

[Zhephyr54]

Proposed change

Add a mention to the SYSTEM_EXEMPT_FROM_DISMISSIBLE_NOTIFICATIONS exemption to restore the pre-Android 14 behavior of persistent notifications (thanks to this comment).

Type of change

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

Checklist

• I have read and followed the documentation guidelines.
• I have verified that my changes render correctly in the documentation.

Additional information

• This PR fixes or closes issue: N/A
• Link to relevant existing code or pull request: N/A