Add note about external domain in requirements

Editor assessment

A minimalist but well-formed commit. `docs: add note about external domain in requirements` uses the right type, starts with a verb in imperative mood, avoids a trailing period, and stays comfortably under the length guideline. The scope is omitted — defensible for repo-wide docs, though something like `docs(guides)` could aid filtering. With no body, readers must open the diff to learn why external domains matter in requirements; a sentence of context would have helped future maintainers. Still, the message is unambiguous, spec-compliant, and correctly signals no user-facing change, so changelog and release tooling can ignore it safely.