Update NEXT_LOCALE cookie note (#22736)
Update NEXT_LOCALE cookie note (#22736)

This updates the `NEXT_LOCALE` cookie docs to mention how it is used for redirecting from `/` more specifically. 

x-ref: https://github.com/vercel/next.js/pull/20045#discussion_r586158535
azukaru/next.js,zeit/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js