[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
Just one `<div>` is enough.

Please also refer to below test cases when you redesign the test:
- https://github.com/w3c/web-platform-tests/blob/master/html/semantics/scripting-1/the-script-element/async_005.htm
- https://github.com/w3c/web-platform-tests/blob/master/dom/nodes/Element-classlist.html
- https://github.com/w3c/web-platform-tests/blob/master/dom/nodes/getElementsByClassName-##