docs(adapter-node): list which env vars custom servers must implement

[vamshikrishnaramasamy]

closes #15734

When you set up a custom server using handler.js from @sveltejs/adapter-node, only the request-handling environment variables (e.g. ORIGIN, BODY_SIZE_LIMIT) actually do anything. The server-lifecycle ones (PORT, HOST, SOCKET_PATH, the various timeouts, and the socket-activation pair) are read by index.js (the default node build server) and have to be implemented manually if you want them in a custom server. The current docs don't mention this, and the existing code example hardcodes port 3000 without flagging that it's ignoring PORT.

Added a note right after the custom-server code example listing both sets explicitly. Cross-checked the variable lists against the source in packages/adapter-node/src/handler.js and packages/adapter-node/src/index.js.

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