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Waiting for a state and waiting for an event are subtly different: for
an event, the waiter *must* be constructed before the event is fired
(or else will spin forever), whereas for a state, a waiter may be
constructed at any point (because the state can be queried).

Update the docs with this differentiation and an example.

Bug: None
Change-Id: Ice984ee861df46d9010986e72779639e6c19e400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2151470
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759698}