[usecase-webapi] Improve Page Visibility usecase

- add description of locking screen/unlocking screen

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: [Andriod]
Test result summary: Pass 1, Fail 0, Blocked 0
Merge pull request #580 from zhangjingx/zhangjingx/misc/pagevisibility

[usecase-webapi] Improve Page Visibility usecase