Update tls.md to mention using the full cert chain

[NathanSweet]

• [X] have read the CONTRIBUTING.md file
• [ ] raised a GitHub issue or discussed it on the projects chat beforehand
• [ ] added unit tests
• [ ] added integration tests
• [X] updated documentation if needed
• [ ] updated CHANGELOG.md

The Tailscale Android client fails silently if the full certificate chain is not provided.