Make @types/next Document generic for defining custom props. (#28429)
Make @types/next Document generic for defining custom props. (#28429)

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- [x] Use a meaningful title for the pull request. Include the name of the package modified.
- [x] Test the change in your own code. (Compile and run.)
- [ ] Add or edit tests to reflect the change. (Run with `npm test`.)
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- [x] Avoid [common mistakes](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/README.md#common-mistakes).
- [x] Run `npm run lint package-name` (or `tsc` if no `tslint.json` is present).

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If changing an existing definition:
- [ ] Provide a URL to documentation or source code which provides context for the suggested changes: <<url here>>
- [x] Increase the version number in the header if appropriate.
- [x] If you are making substantial changes, consider adding a `tslint.json` containing `{ "extends": "dtslint/dt.json" }`.

I'm not familiar with writing these kind of testing code, so I didn't update that. Give me some guidance if necessary. Thanks.
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