system_api: Document potential races in suspend readiness.

Update the comment at the top of powerd's suspend.proto file
to mention that it's possible for a SuspendDone signal to be
emitted while clients are still performing pre-suspend work
in response to the initial SuspendImminent signal. This can
happen when a user closes and then quickly reopens a
device's lid, for instance.

BUG=none
TEST=none

Change-Id: I519b34cc3f367bf6256ee2f104e7ab810e26be58
Reviewed-on: https://chromium-review.googlesource.com/798855


Reviewed-by: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>