Update 'use server' directive explanation

Clarify the usage of 'use server' directive in SolidStart. 

Proposed as it's often misused, as you can see in the wild:

https://github.com/orgs/supabase/discussions/12891

Author: KipzonderKop101

src/routes/solid-start/reference/server/use-server.mdx

@@ -17,14 +17,24 @@ description: >-
   Handle database operations, API calls, and secure logic on the server.
 ---
 
-`"use server"` will enable functions that only run on the server.
+`"use server"` enables functions or files to be executed only on the server. Server functions allow client components to call code that is executed in the server context.
 
 ```tsx
+// Function-level
 const logHello = async (message: string) => {
 	"use server";
 	console.log(message);
 };
 ```
+Or when using at the top of a file.
+```tsx
+// File-level
+"use server";
+
+const logHello = async (message: string) => {
+	console.log(message);
+};
+```
 
 **Note:** `"use server"` functions must be marked async or return a promise.