Docs: Improve JSDoc for `customize-preview-widgets.js`.

Props ireneyoast, andizer, robinvandervliet, boblinthorst.
Fixes #42954.

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git-svn-id: https://core.svn.wordpress.org/trunk@42247 1a063a9b-81f0-0310-95a4-ce76da25c4cd