Treat constructor method as proper constructor of an object

With this minimal class:
```js
/** */
class Foo {
/** */
constructor(bar) {
this.bar = bar;
}
}
```

Running`node_modules/.bin/documentation serve doc.js` produces the following output with 4.0.0-beta.18:

I would expect the constructor not to be displayed as an instance method and it's parameters included in the instantiation example.