More controls beyond the default can be added in a couple of ways and this way, for instance

```javascript
var map = new ol.Map({ /* options */});
map.getControls().extend(...);
```

Extending ol.controls.defaults() is the same as extending map.getControls() after construction.  It is ultimately more key strokes without it being any clearer as to what is happening.  Personally I don't think we should say anything about this in the `MapOptions` documentation.  It would be good to discuss this at either class level or more preferably in some prose/tutorial documentation that goes into more detail than is appropriate for the API documentation.

My preference would be to remove this from here and include it elsewhere.

Also, as written, it sounds like the defaults are being changed (i.e. constructing a second map would pick up the change to the default set of controls)