Clarify `headers` config option description (#33484)
Clarify `headers` config option description (#33484)

Reading over this page, it wasn't immediately clear to me if these were headers which would get read off of the incoming request, or headers to be added to the outgoing response.



## Documentation / Examples

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azukaru/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js