Doc: link to QWindow::requestActivate() in more places

Make it easier to find the function that actually gives focus to
windows that need it.

Change-Id: I0586b89b42a1be260e36e166c81b686e394d8b5e
Reviewed-by: Nicholas Bennett <nicholas.bennett@qt.io>
(cherry picked from commit a94ba94695fe20d0cddc9b33396262ba3ed2b07f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qguiapplication.cpp

@@ -948,6 +948,8 @@ bool QGuiApplicationPrivate::isWindowBlocked(QWindow *window, QWindow **blocking
 /*!
     Returns the QWindow that receives events tied to focus,
     such as key events.
+
+    \sa QWindow::requestActivate()
 */
 QWindow *QGuiApplication::focusWindow()
 {

src/gui/kernel/qwindow.cpp

@@ -1249,6 +1249,8 @@ bool QWindow::isExposed() const
     Typically active windows should appear active from a style perspective.
 
     To get the window that currently has focus, use QGuiApplication::focusWindow().
+
+    \sa requestActivate()
 */
 bool QWindow::isActive() const
 {