x86_64: improve documentation for cpuid vendor
x86_64: improve documentation for cpuid vendor

BUG=None
TEST=None

Change-Id: Id274c361d10be00d5d7279765a2e90ad5ecb4112
Reviewed-on: https://chromium-review.googlesource.com/974314
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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