Inline documentation cleanup for 4.0 audit.

phpDoc tweaks for methods added in [28861]:
* `WP_Customize_Panel` constructor method
* `WP_Customize_Panel::render()` method

See #27406 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29158


git-svn-id: http://core.svn.wordpress.org/trunk@28942 1a063a9b-81f0-0310-95a4-ce76da25c4cd