Updated copy around enabling source maps (#36852)

This PR updates the copy in the source maps section of the docs to reflect the improvement made to webpack that generate source maps.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`

docs/advanced-features/source-maps.md

@@ -4,7 +4,9 @@ description: Enables browser source map generation during the production build.
 
 # Source Maps
 
-Source Maps are enabled by default during development. During production builds, they are disabled as generating source maps can significantly increase build times and memory usage while being generated.
+Source Maps are enabled by default during development. During production builds, they are disabled to prevent you leaking your source on the client, unless you specifically opt in with the configuration flag.
+
+## Configuration flag
 
 Next.js provides a configuration flag you can use to enable browser source map generation during the production build: