Custom mode documentation excludes key information regarding 'active' and 'inactive' states of drawn features

Nobody has claimed this yet.

I've just lot quite a bit of time for something that could be better explained within the Custom Mode documentation.

I've created a custom mode for drawing nodes and lines, and I wanted the "active" styling that is used in the pre-determined modes to be usable as well.

The documentation mentions that using `this.select` will control which features are selected, which does work _internally_, but does not control the 'active' state on the drawn features. I had to dig through the un-minified MapDraw file to discover that the 'active' state is actually to be controlled manually within the `toDisplayFeatures`, like so:

```
DrawLineMode.toDisplayFeatures = function(state, geojson, display) {
geojson.properties.active = !!this.isSelected(geojson.properties.id) + '';
display(geojson);
};
```

It's a simple bit of code obviously, however the documentation could do with a note that the `active` property in the drawn GeoJSON is to be manually controlled within the custom mode, and isn't actually related to the `selected` states.

Locate the Custom Mode documentation and review its sections on this.select, selected features, and toDisplayFeatures; use the un-minified MapDraw file and the DrawLineMode.toDisplayFeatures entry point as references. Done means the documentation clearly distinguishes selected state from the GeoJSON active property and explains the manual active-state handling with the example shown.

Written by the indexing model from the issue text.