[usecase-webapi] Improve Jsenhance usecase

- Add description of animation

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: [Tizen-IVI]
Test result summary: Pass 1, Fail 0, Blocked 0
This is not an improvement either. 'block' is something unclear/duplicate of 'rectangular'. 'visible on the page' is also no need.

Why not redesign this test as: showing a green PASS when all features work, and a red FAIL otherwise.