Add async to middleware docs.

[ncphillips]

I was reading the docs and got nervous. It looked like middleware didn't support async/await. After digging into the examples I found out it is possible.

Not 100% sure if this is the docs change yahs want, but I thought I'd open a PR just incase.

Bug

• Related issues linked using fixes #number
• Integration tests added
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Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
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Documentation / Examples

• Make sure the linting passes by running yarn lint

[molebox]

Hey @ncphillips we do mention this in the middleware docs on the vercel docs site. Good catch that we don't mention it here. I think I'd rather we add the same note (copy) that we have on the vercel docs.

The reason being that if we make the function async in the first example, folks might think that it has to be async, in much the same way that you thought that it doesn't have to be 🙃

If you would like to add it, you can find it here just scroll down a tad.

Thank you!

[ijjk]

Thanks for the PR!