Now Qt 5.3.0 (beta 1) supports tooltip only for Button components. But sometimes we need to setup tooltip for a Label, TextField, TextArea etc. When I tried to search information about this I've found this link: http://doc.qt.digia.com/qtquick-components-symbian-1.1/qml-tooltip.html
I think this component wasn't ported to a new version of QtQuick Components.
Also I've asked this question on the forum: http://qt-project.org/forums/viewthread/40365/