Add dynamic and import note. (#20145)

Add note about import required to be explicit and dynamic required to be top-level module.

docs/advanced-features/dynamic-import.md

@@ -71,6 +71,8 @@ export default Home
 
 `DynamicComponent` will be the default component returned by `../components/hello`. It works like a regular React Component, and you can pass props to it as you normally would.
 
+> **Note**: `import()` needs to be explicitly written without template strings. Furthermore the `import()` has to be inside the `dynamic()` call for Next.js to be able to match webpack bundles / module ids to the specific `dynamic()` call and preload them before rendering. `dynamic()` can't be used inside of React rendering as it needs to be marked in the top level of the module for preloading to work, similar to `React.lazy`.
+
 ## With named exports
 
 If the dynamic component is not the default export, you can use a named export too. Consider the module `../components/hello.js` which has a named export `Hello`: