Document DisplayManager atomicity with AChoreographer

Very early in Android 12's development, AChoreographer's refresh rate
callbacks were updated to dispatch from DisplayManager rather than from
SurfaceFlinger. Now that we've written a test enforcing this behavior,
update the documentation so that API users can depend on more reliable
behavior.

Bug: 168708518
Test: builds
Change-Id: I4f28c7a8d7b92b45dc2b89ba43595a6f0434d09d