Document fullScreenIntent requires IMPORTANCE_HIGH

Adds documentation mentioning that full screen intent requires
not just calling setFullScreenIntent, but also posting the
notification to a channel with importance set to IMPORTANCE_HIGH.
This is currently checked by NotificationInterruptStateProviderImpl,
in shouldLaunchFullScreenIntentWhenAdded.

Bug: 232793248
Test: Only documentation change
Change-Id: Ia9f90464c6f1125339e5d4ebef791c449d01febc