include missing members in onShown info (bug 1625746)

menus.rst

@@ -480,6 +480,20 @@ Fired when a menu is shown. The extension can add, modify or remove menu items a
          :type: (string)
 
 
+      .. api-member::
+         :name: [``selectedFolder``]
+         :type: (:ref:`folders.MailFolder`)
+
+         The selected folder, if the context menu was opened in the folder pane. The :permission:`accountsRead` permission is required.
+
+
+      .. api-member::
+         :name: [``selectedMessages``]
+         :type: (:ref:`messages.MessageList`)
+
+         The selected messages, if the context menu was opened in the message list. The :permission:`messagesRead` permission is required.
+
+
       .. api-member::
          :name: [``selectionText``]
          :type: (string)