Doc: Fix ButtonRole enum docs for QMessageBox and QDialogButtonBox

Multiple topic commands (in this case, \enum) do not work across
different classes.

Reuse the documentation comment via an \include statement instead.

Fixes: QTBUG-78910
Change-Id: Ife83bdc9bbad650835fafc072180d10037648d0a
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/widgets/dialogs/qmessagebox.cpp

@@ -755,6 +755,12 @@ void QMessageBoxPrivate::_q_clicked(QPlatformDialogHelper::StandardButton button
     \sa QDialogButtonBox, {fowler}{GUI Design Handbook: Message Box}, {Standard Dialogs Example}, {Application Example}
 */
 
+/*!
+    \enum QMessageBox::ButtonRole
+
+    \include qdialogbuttonbox.cpp buttonrole-enum
+*/
+
 /*!
     \enum QMessageBox::StandardButton
     \since 4.2

src/widgets/widgets/qdialogbuttonbox.cpp

@@ -522,8 +522,8 @@ QDialogButtonBox::~QDialogButtonBox()
 
 /*!
     \enum QDialogButtonBox::ButtonRole
-    \enum QMessageBox::ButtonRole
 
+//! [buttonrole-enum]
     This enum describes the roles that can be used to describe buttons in
     the button box. Combinations of these roles are as flags used to
     describe different aspects of their behavior.
@@ -546,6 +546,7 @@ QDialogButtonBox::~QDialogButtonBox()
     \omitvalue NRoles
 
     \sa StandardButton
+//! [buttonrole-enum]
 */
 
 /*!