Documented the donutdrilldown chart example

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+/*!
+     \example examples/donutdrilldown
+     \title Donut chart drilldown example
+     \subtitle
+
+     This example shows how to use create a donut drilldown chart using QPieSeries API.
+
+     Let's start by creating a QChartView instance and enabling the Antialiasing on it. Last line enables the animations of the chart.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 1
+
+     PieSeries is used to present the general data.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 2
+
+     Following block of code creates the slices for the mainData QPieSeries.
+     Then for every created slice a new series is created that stores the detailed data.
+     The details series is set to be a donut. Its size is adjusted so that it wraps around the mainData pie.
+     Next two signals from the mainData pie's slices are connected. This is used to keep the mainData slices agligned with their respective details series.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 3
+
+     Set the labels of the mainData to enabled and their postion to Outside.
+     Then add the mainData and detailedData series to the chart.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 4
+
+     Finally the widget is placed in a layout used by the application.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 5
+
+     To show that the detailed data stays aligned with the main data every 2.5 sec. one of the slices is modified.
+     It should be noted that int this example the mainData slices should not be modified directly as the change
+     cannot be applied to the detailed slices without the extra knowledge on how the split looks like.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 6
+
+     When the mainData slice layout is changed the detailed data layout has to be modified accordingly.
+     This is achived by setting the start and end angle of the detailed series.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 7
+
+     Highlight slot selects a random slice for the modification.
+     The slice is set to exploded to notify the user that its going to be changed.
+     The actual data modification is delayed by 1 sec to give a user a chance to focus on the slice.
+
+     Then the slice is modified. Respective mainData slice is modified as well to contain as the value the sum of the detailed series slices values.
+     Finally the slice exploded state is set to false.
+
+     \snippet ../examples/donutdrilldown/widget.cpp 8
+
+*/