Reflect the real return's type of from

Author: Andarist

content/references/api-reference/reactive-utilities/from.mdx

@@ -9,7 +9,7 @@ function from<T>(
           fn: (v: T) => void
         ) => (() => void) | { unsubscribe: () => void };
       }
-): () => T;
+): () => T | undefined;
 ```
 
 A helper to make it easier to interop with external producers like RxJS observables or with Svelte Stores. This basically turns any subscribable (object with a subscribe method) into a Signal and manages subscription and disposal.