docs: update Next.js middleware description (#6155)

* docs: update Next.js middleware description

* Fix grammar in Next.js Edge Runtime description

Correct grammatical error in comparison of Next.js limitations.

* Revise Next.js proxy.ts runtime details

Updated Next.js proxy.ts description to reflect runtime changes.

* removed the not needed '+'

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Co-authored-by: Sarah Gerrard <hello@sarahgerrard.me>