docs: add note clarifying project root in monorepo

[gwkline]

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!

[icyJoseph]

Hi, thank you for this, left a suggestion, that I think we can take in, otherwise, LGTM