CRAS: Update the dbus API document.
CRAS: Update the dbus API document.

Change the dbus API according to feedbacks.

BUG=chromium-os:33666
TEST=none

Change-Id: I56b51e14f062b873a24827a72e6852d4042b12ed
Reviewed-on: https://gerrit.chromium.org/gerrit/45019
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Queue: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>
Tested-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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