docs(api): update stubs documentation

Author: Jamie Stumme

Diff for: docs/api/mount.md

@@ -116,7 +116,7 @@ describe('Foo', () => {
   it('renders a div', () => {
     const wrapper = mount(Foo, {
       stubs: {
-        Bar: '<div class="stubbed" />',
+        Bar: '<div class="stubbed" />', // DEPRECATED
         BarFoo: true,
         FooBar: Faz
       }

Diff for: docs/api/options.md

@@ -208,10 +208,13 @@ const wrapper = mount(WrapperComp).find(ComponentUnderTest)
 
 ## stubs
 
-- type: `{ [name: string]: Component | boolean } | Array<string>`
+- type: `{ [name: string]: Component | string | boolean } | Array<string>`
 
 Stubs child components. Can be an Array of component names to stub, or an object. If `stubs` is an Array, every stub is `<${component name}-stub>`.
 
+Depercation note:
+If declaring an object, string stubs are now deprecated and will be removed in a future version.
+
 Example:
 
 ```js