Add more info about refs

content/warnings/refs-must-have-owner.md

@@ -16,15 +16,45 @@ You are probably here because you got one of the following error messages:
 >
 > addComponentAsRefTo(...): Only a ReactOwner can have refs. You might be adding a ref to a component that was not created inside a component's `render` method, or you have multiple copies of React loaded.
 
-This usually means one of two things:
+This usually means one of three things:
 
+- You are trying to add a `ref` to a functional component.
 - You are trying to add a `ref` to an element that is being created outside of a component's render() function.
 - You have multiple (conflicting) copies of React loaded (eg. due to a misconfigured npm dependency)
 
+## Refs on Functional Components
 
-## Invalid Refs
+If `<Foo>` is a functional component, you can't add a ref to it:
 
-Only a ReactOwner can have refs. This usually means that you're trying to add a ref to a component that doesn't have an owner (that is, was not created inside of another component's `render` method). Try rendering this component inside of a new top-level component which will hold the ref.
+```js
+// Doesn't work if Foo is a function!
+<Foo ref={foo} />
+```
+
+If you need to add a ref to a component, convert it to a class first, or consider not using refs as they are [rarely necessary](/docs/refs-and-the-dom.html#when-to-use-refs).
+
+## Strings Refs Outside the Render Method
+
+This usually means that you're trying to add a ref to a component that doesn't have an owner (that is, was not created inside of another component's `render` method). For example, this won't work:
+
+```js
+// Doesn't work!
+ReactDOM.render(<App ref="app" />, el);
+```
+
+Try rendering this component inside of a new top-level component which will hold the ref. Alternatively, you may use a callback ref:
+
+```js
+let app;
+ReactDOM.render(
+  <App ref={inst => {
+    app = inst;
+  }} />,
+  el
+);
+```
+
+Consider if you [really need a ref](/docs/refs-and-the-dom.html#when-to-use-refs) before using this approach.
 
 ## Multiple copies of React