Docs: neon-animation-finish is not documented

[ebidel]

Not on https://elements.polymer-project.org/elements/neon-animation?active=Polymer.NeonAnimationRunnerBehavior :(

[43081j]

True and it doesn't specify anywhere in the docs what on earth cookie is.

Personally, I've been using it to share data between starting the animation and finishing it, I presumed this was what it was for (seeing as its just e.detail of the event).