docs: document secure, sameSite and __Host- cookie prefix usage

[Tony133]

Proposal:

• Documents that secure, sameSite, and the __Host- cookie name prefix are already fully supported by @fastify/secure-session — no code changes needed, just missing examples in the README.
• Follow-up to Considering adding proprietary cookies good practices as Chrome make big changes on cookies in 2024 #209, where these were requested. Turns out the cookie option is passed straight through to @fastify/cookie's setCookie, which already handles secure and sameSite natively, and the __Host- prefix is just a browser-enforced naming convention with no special handling required in either plugin. The functionality was already there, but the README didn't show how to use it.

Changes:

• Added a bullet in the existing Security section pointing to the new example.
• Added a new Cookie security options (secure, sameSite, and the __Host- prefix) section with two examples: one combining secure + sameSite, and one showing the __Host- prefix setup (secure: true, path: '/', no domain).

Note:

• No functional/code changes — documentation only.