Merge "vibrator: Document the source of always-on source IDs." into rvc-dev am: d3eb82f2a2 am: ac0e5a75ba am: f95fe1432b am: d9771d2d6b am: ce65e23c07

Change-Id: Ia6e61f88878d8bd82fd5edf0f4aa24f9bd6fb839