docs: "Soft" deprecation of ClutterAlpha
docs: "Soft" deprecation of ClutterAlpha

We cannot deprecate ClutterAlpha yet. We cannot also implement
ClutterAlpha in terms of ClutterTimeline, because multiple Alpha
instances can be attached to the same Timeline. So we can start
with a "soft" deprecation: just a warning in the documentation
stating that ClutterAlpha will be deprecated, and removed, in the
future, and that newly-written code should use ClutterTimeline
instead.

djdeath/clutter-android,djdeath/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,djdeath/clutter,heysion/clutter-clone,ebassi/clutter,Distrotech/clutter,collects/clutter,djdeath/clutter,collects/clutter,GNOME/clutter,djdeath/clutter-android,collects/clutter,jigpu/clutter,jigpu/clutter,heysion/clutter-clone,heysion/clutter-clone,heysion/clutter-clone,djdeath/clutter,djdeath/clutter-multithreaded,GNOME/clutter,kerrickstaley/clutter-vala,jigpu/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,GNOME/clutter,Distrotech/clutter,collects/clutter,jigpu/clutter,jigpu/clutter,Distrotech/clutter,ebassi/clutter,Distrotech/clutter,ebassi/clutter,djdeath/clutter,djdeath/clutter,djdeath/clutter-android,ebassi/clutter,ebassi/clutter,Distrotech/clutter,jigpu/clutter,kerrickstaley/clutter-vala,djdeath/clutter-android,djdeath/clutter-multithreaded,GNOME/clutter,djdeath/clutter-multithreaded,GNOME/clutter,Distrotech/clutter,collects/clutter