DOCSP-4441: [Charts] As a user, I see a legend for continuous colors on Geo Heatmap (#145)

* DOCSP-4441: [Charts] As a user, I see a legend for continuous colors on Geo Heatmap

Author: steveren

source/chart-type-reference/geospatial-heatmap.txt

@@ -62,6 +62,14 @@ displayed is based on the aggregated value of those ten data points.
    currently visible on the map, even if there are points outside the
    current view that are more extreme.
 
+Legend
+~~~~~~
+
+Unlike other types of charts, the legend and the chart colors on
+geospatial heatmaps change when the view changes. The color range is
+based on the data in the current view, and may change when you pan or
+zoom the map.
+
 Example
 -------