Fix TextField::placeholderText docs

For usability reasons the placeholder text is kept always visible when
the control is empty, even when focused.

Task-number: QTBUG-36303
Change-Id: I79733b0cf2a48807fb45d5697399f183c2b97f94
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>