Clarify documentation for FULL_WAKE_LOCK

According to the implementation, FULL_WAKE_LOCK is not released
but ignored by the system when the user presses the power button.

Change-Id: Ia9f197ef5a426a68a9c1e6bce04703c5d98f0b6d
Signed-off-by: Sangkyu Lee <sk82.lee@lge.com>