Document LINQ to XML thread safety - #13050

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: dotnet/docs-tools/actions/status-checker@main, step-security/harden-runner@c6295a65d1254861815972266d5933fd6e532bdf. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/