docs: add note clarifying project root in monorepo (#84628)
### What?
- Adding a small note to clarify the meaning of the phrase "project
root" in the documentation for next.config output, specifically in
regards to a monorepo setup
### Why?
- While doing some debugging related to a container that I cannot build
locally for compliance reasons, I had to take shots in the dark to
figure this behavior out 😄 hoping it can help the next soul who finds
themselves in a similar predicament
### How?
- Explain that in monorepos, "project root" does not mean the root of
the monorepo, but instead the root of the nextjs app
First time contributing, so I appreciate any reviews/feedback I can get
here, even for such a small PR. Thanks!
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>
Co-authored-by: Joseph <sephxd1234@gmail.com>
