layout, docs: Add more documentation to LayoutManager
layout, docs: Add more documentation to LayoutManager

The layout manager reference should have some documentation on how
to use a LayoutManager object inside a container and how to implement
a LayoutManager sub-class correctly.

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