Update web-animations pinned keys and description

[ddbeck]

This PR makes some adjustments to the web-animations feature, to more closely align with caniuse and respond to feedback given in #2244.

Specifically, I've made the following changes:

• Pinned the feature's status to a broader part of the API, including getAnimations(), the Animation() constructor, and some common controls
• Changed the description to a capture the whole utility of programmatic animation, instead of fitting specifically to the animate() attribute

The main consequence here is to shift the Baseline low date forward by about 6 months, though still within the same calendar year.

Fixes #2244

Closes #2251