Add child-content note to Download component single-file section

[fern-api]

Added a plain-prose note directly after the single-file code example on the Download component page clarifying that the child of <Download> must be plain text or a <Button> element, and that Markdown link syntax like [click here](.) will not render correctly inside the component.

[github-actions]

🚫 [vale] <Microsoft.Contractions> _{reported by reviewdog 🐶}
Use 'won't' instead of 'will not'.

Suggested change

	The child of `<Download>` must be plain text or a `<Button>` element — Markdown link syntax like `[click here](.)` inside the component will not render correctly.
	The child of `<Download>` must be plain text or a `<Button>` element — Markdown link syntax like `[click here](.)` inside the component won't render correctly.

[github-actions]

🌿 Preview your docs: https://fern-preview-2026-08-20-fern-ai-d8be69-a7f97dfb.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/writing-content/components/download