[docs] Clarify clutter_actor_animate() behaviour
[docs] Clarify clutter_actor_animate() behaviour

When calling clutter_actor_animate() on an actor that is being
currently animated the default behaviour is to update the duration
of the animation; the easing mode; update all the common properties;
and finally add the new properties.

This:

  clutter_actor_animate (actor, 500, CLUTTER_LINEAR,
                         "width", 100,
                         "height", 100,
                         NULL);

  clutter_actor_animate (actor, 250, CLUTTER_EASE_IN_CUBIC,
                         "x", 100,
                         "y", 100,
                         "height", 200,
                         NULL);

Is logically equivalent to:

  clutter_actor_animate (actor, 250, CLUTTER_EASE_IN_CUBIC,
                         "x", 100,
                         "y", 100,
                         "width", 100,
                         "height", 200,
                         NULL);

The documentation of the function should be slightly more verbose
in describing the default behaviour.

GNOME/clutter,collects/clutter,GNOME/clutter,ebassi/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,ebassi/clutter,kerrickstaley/clutter-vala,djdeath/clutter-multithreaded,djdeath/clutter-android,collects/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,jigpu/clutter,djdeath/clutter,tardyp/clutter,Distrotech/clutter,collects/clutter,spatulasnout/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,heysion/clutter-clone,jigpu/clutter,spatulasnout/clutter,GNOME/clutter,nobled/clutter,jigpu/clutter,kerrickstaley/clutter-vala,jigpu/clutter,spatulasnout/clutter,tardyp/clutter,spatulasnout/clutter,tardyp/clutter,heysion/clutter-clone,djdeath/clutter-android,rkudiyarov/clutter_osx,collects/clutter,jigpu/clutter,Distrotech/clutter,rkudiyarov/clutter_osx,jigpu/clutter,rkudiyarov/clutter_osx,heysion/clutter-clone,nobled/clutter,dlespiau/clutter,kerrickstaley/clutter-vala,djdeath/clutter,djdeath/clutter-android,djdeath/clutter,dlespiau/clutter,ebassi/clutter,GNOME/clutter,nobled/clutter,spatulasnout/clutter,djdeath/clutter,djdeath/clutter-multithreaded,dlespiau/clutter,Distrotech/clutter,GNOME/clutter,djdeath/clutter,rkudiyarov/clutter_osx,ebassi/clutter,heysion/clutter-clone,Distrotech/clutter,heysion/clutter-clone,ebassi/clutter,djdeath/clutter,djdeath/clutter-android,collects/clutter