[timeline] Documentation and comment fixes
[timeline] Documentation and comment fixes

The long description of the ClutterTimeline class is very C
developer-oriented. Since many language bindings will refer to
the C API reference we should probably be more verbose and
language agnostic -- at least in the class description.

The methods documentation also requires a little pass to increase
the consistency of the terminology, the grammar and the syntax.

Finally, comments never killed anyone.

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