chore: Updating final Store docs for v3

markdown/dev/reference/api/store/setifunset/en.md

@@ -1,10 +1,23 @@
 ---
-title: setIfUnset
+title: Store.setIfUnset()
 ---
 
+The `Store.set()` method stores the value of `value` in the store under key
+`key`, but only if that key does not already hold a value.
+
+## Signature
+
 ```js
-void store.setIfUnset(string key, mixed value)
+Store store.set(mixed key, mixed value)
+```
+
+<Tip compact>This method is chainable as it returns the `Store` object</Tip>
+
+## Example
+
+```js
+const store = new Store()
+store.set('example', 'Hi there')
+store.setIfUnset('example', 'Hi again') // This has no effect
 ```
 
-Stores the value of `value` in the store under key `key`,
-but only if that key does not already hold a value.