Issue #2879533 by lbaran: Improve Charts code documentation

modules/charts_google/js/charts_google.js

modules/charts_google/src/Charts/GoogleChartsRender.php

modules/charts_google/src/Settings/Google/ChartAxis.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.module

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

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

src/Charts/ChartsRenderInterface.php

src/Charts/ModuleSelector.php

src/Form/ChartsConfigForm.php

src/Plugin/views/display/ChartsPluginDisplayChart.php

src/Plugin/views/style/ChartsPluginStyleChart.php

src/Services/ChartAttachmentService.php

src/Services/ChartAttachmentServiceInterface.php

src/Services/ChartService.php

src/Services/ChartServiceInterface.php

src/Services/ChartsSettingsService.php

src/Services/ChartsSettingsServiceInterface.php