power: Add detailed description to power lock API usage

power: Add detailed description to power lock API usage

device_power_request_lock() follows below rule

1. If the same application requests the same power lock more than twice before it is released or expired,
   the timeout will be updated to the latest requested time.

2. If different applications request the same power lock type,
   the power state timeout will be set to the maximum of the requested timeouts.

Above rule and scenario explanations are added.

Change-Id: Ic0f7a435496913caeee6ae64fee3dc3223e545b0
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>