- Delete the part about removing `[Exposed=Window]` since that's not
necessary and we may want that information there to generate the
Window object.
- Mention adding `#import`s.
- Outline the requirements for the implementation class.
- Mention the non-Event wrapper factories that need to know about
certain types.
I tend to refer to this document every time I add an IDL type so it's
helpful if it's comprehensive.
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