Doc: add more notes about full screen windows on macOS

Fixes: QTBUG-68069
Pick-to: 6.2 5.15
Change-Id: I8fc99f708cfa19a9c8cc8d13f6889549c79dd3b3
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qwindow.cpp

@@ -2215,6 +2215,9 @@ void QWindow::showMaximized()
     Equivalent to calling setWindowStates(Qt::WindowFullScreen) and then
     setVisible(true).
 
+    See the \l{QWidget::showFullScreen()} documentation for platform-specific
+    considerations and limitations.
+
     \sa setWindowStates(), setVisible()
 */
 void QWindow::showFullScreen()

src/widgets/kernel/qwidget.cpp

@@ -2992,9 +2992,9 @@ bool QWidget::isFullScreen() const
 
     Calling this function only affects \l{isWindow()}{windows}.
 
-    To return from full-screen mode, call showNormal().
+    To return from full-screen mode, call showNormal() or close().
 
-    Full-screen mode works fine under Windows, but has certain
+    \note Full-screen mode works fine under Windows, but has certain
     problems under X. These problems are due to limitations of the
     ICCCM protocol that specifies the communication between X11
     clients and the window manager. ICCCM simply does not understand
@@ -3014,7 +3014,14 @@ bool QWidget::isFullScreen() const
     X11 window managers that follow modern post-ICCCM specifications
     support full-screen mode properly.
 
-    \sa showNormal(), showMaximized(), show(), hide(), isVisible()
+    On macOS, showing a window full screen puts the entire application in
+    full-screen mode, providing it with a dedicated desktop. Showing another
+    window while the application runs in full-screen mode might automatically
+    make that window full screen as well. To prevent that, exit full-screen
+    mode by calling showNormal() or by close() on the full screen window
+    before showing another window.
+
+    \sa showNormal(), showMaximized(), show(), isVisible(), close()
 */
 void QWidget::showFullScreen()
 {