tests/futility/test_update.sh: Document test_update function

BUG=b:231084609
BRANCH=None
TEST=None

Change-Id: Id76f2469faa13c136c6ec2761577acec4ad810e5
Signed-off-by: Evan Benn <evanbenn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3831833
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
1 file changed