docs: elaborate on duplicate setHeaders calls (#12886)

documentation/docs/20-core-concepts/20-load.md

@@ -333,7 +333,7 @@ export async function load({ fetch, setHeaders }) {
 }
 ```
 
-Setting the same header multiple times (even in separate `load` functions) is an error — you can only set a given header once. You cannot add a `set-cookie` header with `setHeaders` — use `cookies.set(name, value, options)` instead.
+Setting the same header multiple times (even in separate `load` functions) is an error. You can only set a given header once using the `setHeaders` function. You cannot add a `set-cookie` header with `setHeaders` — use `cookies.set(name, value, options)` instead.
 
 ## Using parent data