Update the web-animations description

[flackr]

Update the web-animations description to include

• Construction of an Animation directly
• Querying and modifying CSS animations and transitions retrieved via getAnimations()

This fixes #2244.

[captainbrosset]

Suggesting some changes to the description.
Also based on my comment here #2244 (comment) I would add the following BCD keys to the feature:

• api.Document.getAnimations
• api.Element.getAnimations
• api.ShadowRoot.getAnimations

[ddbeck]

OK, I've caught up on the discussion here and in the original issue. I've got some ideas for the description (to avoid losing the context of Element and such) and an idea to fix the computed status too. Thank you!

[ddbeck]

Hmm, after reading this it's almost as if we're trying to do too much to tease out the individual pieces of the API. Instead, I'm wondering if we can't give an overview and hint at what things to go look up in the docs. What do you think of something like this?

Suggested change

	description: The `animate()` method, together with the `Animation` interface, animates an element over time, possibly synchronizing the animations of multiple elements. The `getAnimations()` method queries and modifies current CSS animations, transitions, and web animations.
	description: The `animate()` method of `Element` objects, the `document.getAnimations()` method, and the `Animation` interface animate elements over time and coordinate elements' scripted animations, CSS animations, and transitions.

[ddbeck]

It's out of range of the diff, so I can't directly suggest it, but to match the description and fully resolve #2244 we also need to change which keys drive the headline status. This will also line us more closely with caniuse.

You'll need this in the current YAML file:

status:
  compute_from:
    - api.Element.animate
    - api.Animation
    - api.Document.getAnimations

and you'll need to run npm install and npm run dist and commit the results.