Issue #2879533 by lbaran: Improve Charts code documentation

charts.api.php

includes/charts.pages.inc

modules/charts_c3/charts_c3.module

modules/charts_c3/src/Charts/C3ChartsRender.php

modules/charts_c3/src/Plugin/chart/CThreeCharts.php

modules/charts_c3/src/Settings/CThree/ChartData.php

modules/charts_c3/src/Settings/CThree/ChartType.php

modules/charts_google/js/charts_google.js

modules/charts_google/src/Charts/GoogleChartsRender.php

modules/charts_google/src/Plugin/chart/GoogleCharts.php

modules/charts_google/src/Settings/Google/ChartArea.php

modules/charts_google/src/Settings/Google/ChartAxes.php

modules/charts_google/src/Settings/Google/ChartType.php

modules/charts_google/src/Settings/Google/GoogleOptions.php

modules/charts_google/src/Settings/Google/HorizontalAxis.php

modules/charts_google/src/Settings/Google/VerticalAxis.php

modules/charts_highcharts/charts_highcharts.install

modules/charts_highcharts/charts_highcharts.module

...s/charts_highcharts/src/Charts/HighchartsChartsRender.php

modules/charts_highcharts/src/Plugin/chart/Highchart.php