Improve wxMenu::Break() documentation and show it in the sample

Document that this method only actually does something in wxMSW.

Demonstrate the use of it in the menu sample.

See #18692.

samples/menu/menu.cpp

@@ -592,7 +592,15 @@ MyFrame::MyFrame()
                      "Enable or disable the last menu item", true);
     menuMenu->Append(Menu_Menu_Check, "&Check menu item\tAlt-C",
                      "Check or uncheck the last menu item", true);
+
+    // Show the effect of Break(). As wxMSW is the only port in which calling
+    // it actually does something, insert a separator under the other platforms.
+#ifdef __WXMSW__
+    menuMenu->Break();
+#else
     menuMenu->AppendSeparator();
+#endif
+
     menuMenu->Append(Menu_Menu_GetInfo, "Get menu item in&fo\tAlt-F",
                      "Show the state of the last menu item");
 #if wxUSE_TEXTDLG