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

See #26870.



git-svn-id: https://develop.svn.wordpress.org/trunk@26987 602fd350-edb4-49c9-b593-d223f7449a82
Correct a type.

See #26870.



git-svn-id: https://develop.svn.wordpress.org/trunk@26988 602fd350-edb4-49c9-b593-d223f7449a82