Add `async` to middleware docs. (#31356)
Add `async` to middleware docs. (#31356)

* Add `async` to middleware docs. 

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.

* Add middleware API note

Co-authored-by: JJ Kasper <7323a5431d1c31072983a6a5bf23745b655ddf59@jjsweb.site>
azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js