docs: add explicit example for opting package out of --exclude-newer (#18803)

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## Summary

Add the explicit configuration for opting out a package out of
`--exclude-newer`. The docs mention this from
https://github.com/astral-sh/uv/pull/16854, but the actual
pyproject.toml configuration was missing. I found it from
https://github.com/astral-sh/uv/issues/12449#issuecomment-4170155721,
but this should be in the docs.

## Test Plan

uv run --only-group docs mkdocs serve -f mkdocs.yml

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

docs/concepts/resolution.md

@@ -721,8 +721,16 @@ Values may also be specified for specific packages, e.g.,
 exclude-newer-package = { setuptools = "2006-12-02T02:07:43Z" }
 ```
 
-The same flag also accepts `<package>=false` to opt a package out of the `--exclude-newer`
-restriction, e.g., to allow resolving packages from an index that does not publish upload times.
+The package option also accepts `<package>=false` to opt a package out of the restriction, e.g.,
+`--exclude-newer-package setuptools=false`, or:
+
+```pyproject.toml
+[tool.uv]
+exclude-newer-package = { setuptools = false }
+```
+
+This is useful to temporarily use a newer version of package or to allow resolving a package from an
+index that does not publish upload times.
 
 Package-specific values will take precedence over global values.