Docs: Improve JS Docs for `custom-header.js`.

The `custom-header.js` file is only used on the `wp-admin/themes.php?page=custom-header` page. As that page is no longer linked from the UI the functions in the file are marked deprecated.

Props nicollle, jipmoors, diedeexterkate.
Fixes #42678.

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git-svn-id: https://core.svn.wordpress.org/trunk@42214 1a063a9b-81f0-0310-95a4-ce76da25c4cd