Note User Agent and AT Filtering of ariaNotify notifications.

[janewman]

Originally brought up in #2577 (comment)

Discussed in the Jan 22nd 2026 AriaWG call, #2577 (comment)

RESOLVED: We will land AriaNotify and deal with the issue about what user agents should do with notifications from background tabs in a follow up, or what note to add to help authors understand filtering, in a follow up PR.

[smockle]

This is related to #2499: This issue tracks adding a note saying ‘user agents and AT could filter ariaNotify notifications’; that issue tracks the filtering itself.