Make comments more clear in read/throttle event proto defs

BUG=none
TEST=Unit tests pass

Change-Id: I6b60e8b674c632043ca49e5c33c161203627a8f1
Signed-off-by: Simon Que <sque@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/189109
342 files changed