Inline documentation cleanup for 4.0 audit.

phpDoc tweaks for methods, properties, and filters added in [29051]:
* `WP_Customize_Widgets::is_widget_rendered()` method
* `WP_Customize_Widgets::is_sidebar_rendered()` method

See #27993 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29159


git-svn-id: http://core.svn.wordpress.org/trunk@28943 1a063a9b-81f0-0310-95a4-ce76da25c4cd