axisTitleData missing in LineChartData

In the [doc for the `LineChartData`
](https://github.com/imaNNeoFighT/fl_chart/blob/master/repo_files/documentations/line_chart.md#linechartdata) as well as in the doc comment for it, the `axisTitleData` is mentioned.

But `LineChartData` has no `axisTitleData`, which probably should be there.

#### Code to reprouce
```dart
class Test extends StatelessWidget {
const Test({Key? key}) : super(key: key);

@override
Widget build(BuildContext context) {
return LineChart(
LineChartData(
axisTitleData: AxisTitles(),
),
);
}
}
```

Start with the LineChartData documentation and its doc comment, then compare both with the current LineChartData API used by the reproduction. Determine whether the missing axisTitleData property or the outdated references should be corrected, and verify that the documented example matches the available API.

Written by the indexing model from the issue text.