vboot/ui: improve menu navigation docstrings

BUG=b:146399181
TEST=make clean && make runtests
BRANCH=none

Change-Id: I8cd47503384a8b8f48ccbc12fa6f24c71d02b755
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2210016
Reviewed-by: Hsuan Ting Chen <roccochen@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
6 files changed