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!

---------

Co-authored-by: Joseph <[email protected]>
Co-authored-by: Joseph <[email protected]>

Author: gwkline

docs/01-app/03-api-reference/05-config/01-next-config-js/output.mdx

@@ -79,6 +79,9 @@ module.exports = {
 
 - There are some cases in which Next.js might fail to include required files, or might incorrectly include unused files. In those cases, you can leverage `outputFileTracingExcludes` and `outputFileTracingIncludes` respectively in `next.config.js`. Each option accepts an object whose keys are **route globs** (matched with [picomatch](https://www.npmjs.com/package/picomatch#basic-globbing) against the route path, e.g. `/api/hello`) and whose values are **glob patterns resolved from the project root** that specify files to include or exclude in the trace.
 
+> **Good to know**:
+> In a monorepo, `project root` refers to the Next.js project root (the folder containing next.config.js, e.g., packages/web-app), not necessarily the monorepo root.
+
 ```js filename="next.config.js"
 module.exports = {
   outputFileTracingExcludes: {