docs: add note about waiting for asynchronous trigger event to be handled

Author: Samuli

docs/api/wrapper/emitted.md

@@ -14,6 +14,8 @@ const wrapper = mount(Component)
 wrapper.vm.$emit('foo')
 wrapper.vm.$emit('foo', 123)
 
+await wrapper.vm.$nextTick()  // Wait until $emits have been handled
+
 /*
 wrapper.emitted() returns the following object:
 {

docs/api/wrapper/trigger.md

@@ -1,6 +1,6 @@
 ## trigger
 
-Triggers an event on the `Wrapper` DOM node.
+Triggers an event asynchronously on the `Wrapper` DOM node.
 
 `trigger` takes an optional `options` object. The properties in the `options` object are added to the Event.
 
@@ -31,6 +31,8 @@ wrapper.trigger('click', {
   ctrlKey: true // For testing @click.ctrl handlers
 })
 
+await wrapper.vm.$nextTick()  // Wait until trigger events have been handled
+
 expect(clickHandler.called).toBe(true)
 ```