Document parenting behaviour of delegates in QML ListView and Component.

ListView:
As delegates are instantiated in ListView, they are parented to views
contentItem. This is now explicitly stated in the documentation.

Component:
Components are not derived from Items, so they cannot hook to anchors.

Task-number: QTBUG-24822
Change-Id: I09e04fbcc01b2858c039d22f9600ef201a9a91ab
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>