doc: close event is always emitted

Test case:

```js
var mongodb = require('mongodb');

mongodb.MongoClient.connect('mongodb://localhost/test', function (err, db) {
  if(err) { throw err; }

  db.on('close', function () { console.log('CLOSE EVENT'); });
  db.close(function () { console.log('CLOSE CALLBACK'); });
});
```

Output:

```text
CLOSE CALLBACK
CLOSE EVENT
```