docs: Type narrowing using LayoutProps, PageProps (#83692)
Make a couple of clarifications:
- type resolved by the helpers
- ways to narrow down to a narrower union
---------
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Rich Haines <hello@richardhaines.dev>
