Update policy description for SystemUse24HourClock policy.

This clarifies the policy description to agree with the implementation
and describe the default behavior.

BUG=
TEST=None, doc update only.

Review URL: https://chromiumcodereview.appspot.com/23125005

git-svn-id: http://src.chromium.org/svn/trunk/src/chrome/browser/policy/proto@218753 4ff67af0-8c30-449e-8e8b-ad334ec8d88c