Docs: Improve AnimationMixer page.

[trusktr]

and tip at using it when there is a single root upon disposing a mixer. (Let me know if this doc update is correct, it wasn't clear to me from the doc, which is why I'd like to clarify it)

Related issue: N/A

Description

Make it a little easier to understand how to clean up.

This contribution is funded by Lume (3D HTML elements)

[trusktr]

I'm making this doc update based on code like this (wondering if it is correct):

                // After loading an asset with GLTFLoader, make the mixer.
                let mixer = new AnimationMixer(model.scene)

                // Play all animations by default.
                for (const anim of model.animations) {
                    const action = mixer.clipAction(anim)
                    action.play()
                }

                // ... later when ready to dispose animations (f.e. keeping the model, but applying different animations)
                mixer.stopAllAction()
                mixer.uncacheRoot(model.scene)
                
                // ... even later, create a new AnimationMixer with new animations, using the same model.scene root.
                // ... even later, dispose animations the same way, etc