crosvm: fix documentation for SystemEvent
crosvm: fix documentation for SystemEvent

The documentation for the SystemEvent exit reason needs to specify
the positions for the two fields.

BUG=chromium:797868
TEST=./build_test passes on all architectures

Change-Id: Ida98ba4a6b383a1c10fa48356decc1c5264999ec
Reviewed-on: https://chromium-review.googlesource.com/986721
Commit-Ready: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Tested-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm