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_google/src/Settings/Google/ChartAxes.php

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

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

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

src/Plugin/chart/ChartInterface.php

src/Plugin/chart/ChartManager.php

src/Util/Util.php