Show how the displayName can be accessed

This has been ask many times, for example in #2285.
I like the effort, but...

This section is discussing the spec, where `displayName` must be specified directly, not as a property of `constructor`. ie

```js
React.createClass({
  displayName: 'MyComponent',
  ...
})
```
not

```js
React.createClass({
  constructor: {
    displayName: 'MyComponent'
  },
  ...
})
```

The latter seems like an easy mistake to make given the way you have this presented. I would instead leave this as is and add a separate paragraph or note mentioning how it can be accessed for reading.