QGraphicsItem: clarify the documentation about caching modes and update()

It is possible that Qt calls paint() several times for an item that
has caching enabled, even without any explicit call to update(). There
are various reasons why that is possible (memory pressure, item gets
transformed, etc.); the important part is that

1) the user must not rely into "caching enabled" = "1 paint() call"
2) the user must always draw the same content from within paint(),
unless update() was called before that call to paint().

Task-number: QTBUG-18410
Change-Id: I2c8d77e6e11aaceffc9a21003dd3f4cc46edd582
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>

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