docs: describe mock result behavior with promises (#3635)

docs/api/mock.md

@@ -269,7 +269,7 @@ This is an array containing all values, that were `returned` from the function.
 - `'return'` - function returned without throwing.
 - `'throw'` - function threw a value.
 
-The `value` property contains returned value or thrown error.
+The `value` property contains the returned value or thrown error. If the function returned a promise, when it resolves the `value` property will become the value the promise resolved to.
 
 ```js
 const fn = vi.fn()
@@ -318,4 +318,4 @@ const a = new Spy()
 Spy.mock.instances[0] !== a
 Spy.mock.results[0] === a
 ```
-:::
+:::