Add documentation about property value modifiers

Add documentation about property value modifiers

Previously, property value modifiers were only documented in QtQuick,
when really they are a QtQml module / QML language feature.

This commit also improves the QtQuick documentation for convenience
types, and rearranges some links and content for improved consistency.

Change-Id: Ic4dbe0b8bb85bdfbb8ac2f336c13c2960f8d1842
Reviewed-by: Bea Lam <bea.lam@nokia.com>