Docs: clarify requestAnimationFrame speed

[Kos]

It's not strictly true that requestAnimationFrame runs at 60fps - it depends on the device.
Assuming 60fps will make your app misbehave on computers with 75hz or 144hz screens.
This is an early tutorial, but let's at least give an early heads up that there's a gotcha here.

[Kos]

I'd look for a good place to provide example code how to avoid the problem by using the time parameter passed to requestAnimationFrame, do you think we should have this too in the docs? FAQ? separate page?

[looeee]

I think that having a short tutorial titled "the render loop" in the getting started section of the manual would be a useful addition, since this programming pattern is used in nearly all three.js projects.

[mrdoob]

Thanks!