Add documentation for NOTIFICATION_SHOW_ONLY_UNSEEN_NOTIFICATIONS

Add documentation for NOTIFICATION_SHOW_ONLY_UNSEEN_NOTIFICATIONS.
NOTIFICATION_SHOW_ONLY_UNSEEN_NOTIFICATIONS has three states:
0: unset_off, which is the default value for phones
1: on
2: off

Bug: 391889319
Flag: DOCS_ONLY
Test: manual
Change-Id: Ie7e66116098fdd64d66802e28829ecda54b2ea8e