naughty.widget.message: Update the property documentation.

lib/naughty/widget/message.lua

@@ -25,6 +25,7 @@ local message = {}
 --- The attached notification.
 -- @property notification
 -- @tparam naughty.notification notification
+-- @propertydefault This is usually set in the construtor.
 -- @propemits true false
 
 function message:set_notification(notif)
@@ -86,8 +87,6 @@ local function new(args)
     return tb
 end
 
---@DOC_widget_COMMON@
-
 --@DOC_object_COMMON@
 
 return setmetatable(message, {__call = function(_, ...) return new(...) end})