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

See #26870.


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Correct a type.

See #26870.


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Disambiguate `this` and use the `@this` annotation in `media-editor.js`. This will become way more necessary in other places, but a few examples here show how confusing it can be. 

See #26870.


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Remove debug cruft.

Props nacin.
See #26870.


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Make some `@param` types more specific in `media-models.js`.

See #26870.


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