docs(chart): clarify donut multiple series

Author: marin-bratanov

components/chart/data-bind.md

@@ -38,7 +38,8 @@ You can use only series with the same general layout in a single chart. You cann
 * Bar and Column charts have a different layout and the rendering will depend on the first declared series.
 * Scatter and ScatterLine series can be used together.
 * Bubble charts cannot be used with other chart types because they have a very distinct layout due to the Size dimension.
-* Pie and Donut charts will render only one series per chart and so only one can be used at a time.
+* Pie charts will render only one series per chart and so only one can be used at a time.
+* Donut charts can only have donut series in them.