Docs: Improve `AnimationMixer` page. (#27914)

* Update AnimationMixer.html: describe that `AnimationMixer.stopAllAction()` will call `AnimationAction.stop()` on all actions, and tip at using it when there is a single root upon disposing a mixer.

* Update AnimationMixer.html

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Co-authored-by: Michael Herzog <[email protected]>

docs/api/en/animation/AnimationMixer.html

@@ -102,7 +102,8 @@
 		<p>
 			Deallocates all memory resources for a root object. Before using this
 			method make sure to call [page:AnimationAction.stop]() for all related
-			actions.
+			actions or alternatively [page:.stopAllAction]() when the mixer operates 
+			on a single root.
 		</p>
 
 		<h3>[method:undefined uncacheAction]([param:AnimationClip clip], [param:Object3D optionalRoot])