naughty.container.background: Update the property documentation.

lib/naughty/container/background.lua

@@ -44,6 +44,7 @@ end
 --- The attached notification.
 -- @property notification
 -- @tparam naughty.notification notification
+-- @propertydefault This is usually set in the construtor.
 -- @propemits true false
 
 function background:set_notification(notif)
@@ -101,8 +102,6 @@ local function new(args)
     return bg
 end
 
---@DOC_widget_COMMON@
-
 --@DOC_object_COMMON@
 
 return setmetatable(background, {__call = function(_, ...) return new(...) end})