docs: fix type signature for StartStopNotifier

It used the Subscriber type to represent the set callback and the
Unsubscriber to represent the cleanup callback. But the names made
it confusing what it was for.

src/runtime/store/index.ts

@@ -12,8 +12,15 @@ export type Updater<T> = (value: T) => T;
 /** Cleanup logic callback. */
 type Invalidator<T> = (value?: T) => void;
 
-/** Start and stop notification callbacks. */
-export type StartStopNotifier<T> = (set: Subscriber<T>) => Unsubscriber | void;
+/** 
+ * Start and stop notification callbacks.
+ * This function is called when the first subscriber subscribes.
+ * 
+ * @param {(value: T) => void} set Function that sets the value of the store.
+ * @returns {void | (() => void)} Optionally, a cleanup function that is called when the last remaining
+ * subscriber unsubscribes.
+ */
+export type StartStopNotifier<T> = (set: (value: T) => void) => void | (() => void);
 
 /** Readable interface for subscribing. */
 export interface Readable<T> {
@@ -48,7 +55,7 @@ const subscriber_queue = [];
 /**
  * Creates a `Readable` store that allows reading by subscription.
  * @param value initial value
- * @param {StartStopNotifier}start start and stop notifications for subscriptions
+ * @param {StartStopNotifier} [start] 
  */
 export function readable<T>(value?: T, start?: StartStopNotifier<T>): Readable<T> {
 	return {
@@ -59,7 +66,7 @@ export function readable<T>(value?: T, start?: StartStopNotifier<T>): Readable<T
 /**
  * Create a `Writable` store that allows both updating and reading by subscription.
  * @param {*=}value initial value
- * @param {StartStopNotifier=}start start and stop notifications for subscriptions
+ * @param {StartStopNotifier=} start
  */
 export function writable<T>(value?: T, start: StartStopNotifier<T> = noop): Writable<T> {
 	let stop: Unsubscriber;