Improve documentation of QWinTaskbarButton::setWindow().

Mention that QWidget::windowHandle() may return 0 until the widget
is shown.

Task-number: QTBUG-47035
Change-Id: Ied8ceb6c1652f5f416a51dc6378599d5efe85420
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>

src/winextras/doc/qtwinextras.qdocconf

@@ -13,7 +13,7 @@ headerdirs = .. ../../imports/winextras
 sourcedirs = .. ../../imports/winextras
 imagedirs += images
 
-depends += qtcore qtgui qtdoc
+depends += qtcore qtgui qtdoc qtwidgets
 
 qhp.projects = QtWinExtras
 

src/winextras/qwintaskbarbutton.cpp

@@ -71,6 +71,10 @@ QT_BEGIN_NAMESPACE
 
     \snippet code/taskbar.cpp taskbar_cpp
 
+    \note QWidget::windowHandle() returns a valid instance of a QWindow only
+    after the widget has been shown. It is therefore recommended to delay the
+    initialization of the QWinTaskbarButton instances until QWidget::showEvent().
+
     \note The class wraps API only available since Windows 7. Instantiating it
     on Windows XP or Windows Vista causes a runtime warning.