doc/release: Use twine check --strict to fail on warnings

This was suggested by webknjaz.[1]

[1]: pypa/packaging.python.org#1535 (comment)

Author: 0cjs

doc/release.md

@@ -29,7 +29,7 @@ Release Process
    - `pip install build twine`.
    - `rm -f dist/*`
    - `pyproject-build`.
-   - `twine check dist/*`
+   - `twine check --strict dist/*`  (the strict opt. fails on warnings)
    - Fix anything broken.
 
 3. Do the release