QGraphicsTextItem: document that adjustSize can center/right align text

Unlike QTextEdit, there's no implicit width for a QGraphicsTextItem,
meaning any rich text that is supposed to be centered or right aligned
will not be unless the user explicitly sets the width.

There's also another possibility: calling adjustSize() manually.
Document that.

Task-number: QTBUG-312
Change-Id: I83023269f4c63643e8224887efe2143dd1bb1908
Reviewed-by: Andy Shaw <andy.shaw@digia.com>