Elaborate on JSDoc blocks for `media-editor.js`.

See #26870.


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Correct a type.

See #26870.


Built from https://develop.svn.wordpress.org/trunk@26988


git-svn-id: http://core.svn.wordpress.org/trunk@26865 1a063a9b-81f0-0310-95a4-ce76da25c4cd