[Notifications] Improve callback documentation for getDisplayed

Change-Id: I69d99479777347a8f9036792a03e6ad096970f96
Reviewed-on: https://chromium-review.googlesource.com/874070
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530161}
1 file changed