Document rooting mechanisms and when/how to use them

[jdm]

• rooted!/&RootedGuard<T>/impl impl js::gc::Rootable for T
• auto_rooter!/&CustomAutoRooterGuard<T>
• DomRoot<T>
• Trusted<T>
• Dom<T>
• Heap<T>
• Handle<T>/MutableHandle<T>