Issue #2879533 by lbaran, andileco: Improve Charts code documentation

charts.api.php

includes/charts.pages.inc

modules/charts_c3/README.txt

modules/charts_google/README.txt

modules/charts_highcharts/README.txt

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

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

modules/charts_highcharts/src/Settings/Highcharts/Chart.php

...charts_highcharts/src/Settings/Highcharts/ChartLegend.php

.../charts_highcharts/src/Settings/Highcharts/ChartTitle.php

.../charts_highcharts/src/Settings/Highcharts/Highcharts.php

...charts_highcharts/src/Settings/Highcharts/PlotOptions.php

..._highcharts/src/Settings/Highcharts/PlotOptionsSeries.php

...s/src/Settings/Highcharts/PlotOptionsSeriesDataLabels.php

...les/charts_highcharts/src/Settings/Highcharts/Tooltip.php