Document easing function requirements

[stevage]

https://www.mapbox.com/mapbox-gl-js/api/#AnimationOptions says:

easing (Function) : The animation's easing function.

But it doesn't say what arguments will be passed, nor what it should return.

I took a punt that it might be f(t, b, c, d) (from here) but that didn't seem to be right.

[stevage]

Ok, it looks like it's just f(t), where t is between 0 and 1, and you return a value between 0 and 1.

It's redundantly referred to at https://www.mapbox.com/mapbox-gl-js/api/#Map#flyTo as well.

[mourner]

Correct! Want to submit a PR that improves the docs on this?

[stevage]

Yeah, seems like something I could do - especially since you have such wonderful meta-documentation.