[docs] More information on animation queueing

[docs] More information on animation queueing

Queuing an animation on an actor cannot be done from within the
::completed signal handler, because we guarantee that the Animation
instance is valid and attached to the actor it animates for the
whole duration of the signal emission chain.

In order to queue animations you have to install an idle handler
on the main loop, and call clutter_actor_animate() inside it.

The documentation should be more clear about this caveat in the
memory management of ClutterAnimations created by the animate()
family of functions.