Clarify serialization behavior of "use cache" (vercel#72286)

Maybe not ideal wording but this should be more correct. See
vercel#72081 (comment).

---------

Co-authored-by: Delba de Oliveira <[email protected]>
Co-authored-by: JJ Kasper <[email protected]>

docs/02-app/02-api-reference/01-directives/use-cache.mdx

@@ -58,7 +58,9 @@ export async function getData() {
 ## Good to know
 
 - `use cache` is an experimental Next.js feature, and not a native React feature like [`use client`](/docs/app/api-reference/directives/use-client) or [`use server`](/docs/app/api-reference/directives/use-server).
-- The arguments (or props) passed to a cached function must be [serializable](https://react.dev/reference/rsc/use-server#serializable-parameters-and-return-values) by React. This means they can be converted to a format like JSON. Non-serializable props like functions or React elements cannot be used as arguments.
+- Any [serializable](https://react.dev/reference/rsc/use-server#serializable-parameters-and-return-values) arguments (or props) passed to the cached function, as well as any serializable values it reads from the parent scope, will be converted to a format like JSON and automatically become a part of the cache key.
+- Any non-serializable arguments, props, or closed-over values will turn into opaque references inside the cached function, and can be only passed through and not inspected nor modified. These non-serializable values will be filled in at the request time and won't become a part of the cache key.
+  - For example, a cached function can take in JSX as a `children` prop and return `<div>{children}</div>`, but it won't be able to introspect the actual `children` object.
 - The return value of the cacheable function must also be serializable. This ensures that the cached data can be stored and retrieved correctly.
 - Functions that use the `use cache` directive must not have any side-effects, such as modifying state, directly manipulating the DOM, or setting timers to execute code at intervals.
 - If used alongside [Partial Prerendering](/docs/app/building-your-application/rendering/partial-prerendering), segments that have `use cache` will be prerendered as part of the static HTML shell.