Clarify next/image usage with next export based on feedback. (#33555)

[See feedback here](#19065 (reply in thread)) for how we could improve clarity on these docs.

Author: leerob

docs/api-reference/next/image.md

@@ -278,7 +278,7 @@ The following Image Optimization cloud providers are included:
 
 If you need a different provider, you can use the [`loader`](#loader) prop with `next/image`.
 
-> The `next/image` component's default loader is not supported when using [`next export`](/docs/advanced-features/static-html-export.md). However, other loader options will work.
+> Images can not be optimized at build time using [`next export`](/docs/advanced-features/static-html-export.md), only on-demand. To use `next/image` with `next export`, you will need to use a different loader than the default. [Read more in the discussion.](https://github.com/vercel/next.js/discussions/19065)
 
 > The `next/image` component's default loader uses [`squoosh`](https://www.npmjs.com/package/@squoosh/lib) because it is quick to install and suitable for a development environment. When using `next start` in your production environment, it is strongly recommended that you install [`sharp`](https://www.npmjs.com/package/sharp) by running `yarn add sharp` in your project directory. This is not necessary for Vercel deployments, as `sharp` is installed automatically.