docs: Explain enums for properties and signals
docs: Explain enums for properties and signals

Add some more explanatory comments about the PROP_ and
signals enums.

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