Document QScreen::grabWindow high-DPI behavior

Change-Id: Ib32510ff30e907365e64921fda14e686a495c77c
Reviewed-by: Tor Arne Vestbø <[email protected]>

src/gui/kernel/qscreen.cpp

@@ -701,6 +701,11 @@ void QScreenPrivate::updatePrimaryOrientation()
     border of the window. If \a height is negative, the function
     copies everything to the bottom of the window.
 
+    The offset and size arguments are specified in device independent
+    pixels. The returned pixmap may be larger than the requested size
+    when grabbing from a high-DPI screen. Call QPixmap::devicePixelRatio()
+    to determine if this is the case.
+
     The window system identifier (\c WId) can be retrieved using the
     QWidget::winId() function. The rationale for using a window
     identifier and not a QWidget, is to enable grabbing of windows