Add initial JSDoc blocks to `media-editor.js`. The initial blocks are a...

Add initial JSDoc blocks to `media-editor.js`. The initial blocks are a baseline to work from and invite future iterations. Initial commit is to avoid commits this large in the future.

See #26870.


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git-svn-id: https://core.svn.wordpress.org/trunk@26861 1a063a9b-81f0-0310-95a4-ce76da25c4cd