Doc: Explain emission of QAction::changed on calling QAction::toggle

Extend description of QAction::checked() and QAction::toggled(), as
it is not clear from the documentation that calling QAction::toggle
causes the emission of a QAction::changed signal.

Task-number: QTBUG-68213
Change-Id: I5afa4d9a6e6f001dff81271d1e34d0c515da3c24
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/kernel/qaction.cpp

@@ -968,7 +968,10 @@ void QAction::toggle()
     Only checkable actions can be checked.  By default, this is false
     (the action is unchecked).
 
-    \sa checkable
+    \note The notifier signal for this property is toggled(). As toggling
+    a QAction changes its state, it will also emit a changed() signal.
+
+    \sa checkable, toggled()
 */
 void QAction::setChecked(bool b)
 {
@@ -1190,7 +1193,8 @@ void QAction::activate(ActionEvent event)
 
     This signal is emitted whenever a checkable action changes its
     isChecked() status. This can be the result of a user interaction,
-    or because setChecked() was called.
+    or because setChecked() was called. As setChecked() changes the
+    QAction, it emits changed() in addition to toggled().
 
     \a checked is true if the action is checked, or false if the
     action is unchecked.