Introduce documentation for the `$id_base`, `$name`, `$widget_options`, `$control_options`, `$number`, `$id`, and `$updated` properties in `WP_Widget'.

See #30315.


git-svn-id: https://develop.svn.wordpress.org/trunk@30773 602fd350-edb4-49c9-b593-d223f7449a82