Update doc for audio-coupled-haptic related stuff.

When using audio-coupled-haptic playback, the haptic channels will be
muted if the app doesn't have the vibrate permission. Update the doc to
state that apps should hold vibrate permission to make
audio-coupled-haptic works as expected.

Bug: 236586224
Test: make
Change-Id: I69c1332d39f997a61f3a969d652144d829de9659