docs: Explain why destroy() is implemented
docs: Explain why destroy() is implemented

As destroy() is Clutter-specific and not generic
GObject code, explain why we implement it
(rather than dispose()).

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