Mention `process.env` for config env vars

Editor assessment

`docs: mention process.env for config env vars` names the exact mechanism added to the documentation and the topic it clarifies; nothing requires the diff. Docs is the correct type, and no scope is needed for a guide note. No body quotes the added sentence — a nice-to-have — but for a one-topic documentation addition the subject suffices. The trailing PR reference is routine. Imperative verb, no trailing period, compact. Clear, correct, and self-contained: a publishable example with only the absent quote as a minor gap.