add documentation for cluster properties (#7000)

src/style-spec/reference/v8.json

@@ -361,7 +361,7 @@
     "cluster": {
       "type": "boolean",
       "default": false,
-      "doc": "If the data is a collection of point features, setting this to true clusters the points by radius into groups."
+      "doc": "If the data is a collection of point features, setting this to true clusters the points by radius into groups. Cluster groups become new `Point` features in the source with additional properties:\n * `cluster` Is `true` if the point is a cluster \n * `cluster_id` A unqiue id for the cluster to be used in conjunction with the [cluster inspection methods](https://www.mapbox.com/mapbox-gl-js/api/#geojsonsource#getclusterexpansionzoom)\n * `point_count` Number of original points grouped into this cluster\n * `point_count_abbreviated` An abbreviated point count"
     },
     "clusterRadius": {
       "type": "number",