Add some comments to Settings.in and RuntimeEnabledFeatures.in

See https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/984HMZyFQNo
for discussion.

BUG=None

Review URL: https://codereview.chromium.org/26289003

git-svn-id: svn://svn.chromium.org/blink/trunk@159342 bbb929c8-8fbe-4397-9dbb-9b2b20218538