Add documentation for structural option in animateCss service

[sevdog]

The structural option for $animateCss service is undocumented on the service page (https://docs.angularjs.org/api/ngAnimate/service/$animateCss), but is used in the animation examples (https://docs.angularjs.org/api/ngAnimate).

To find out what this option do you must go to the source code of the service.
This feature should be documented for better knowledge.

[Narretz]

Would you like to open a PR with the changes?

[sevdog]

I will as soon as I have time.