Update reference docs for geocentroid aggregation.
This includes a link to the Wikipedia page explaining what a centroid
is.
Closes
[[search-aggregations-metrics-geocentroid-aggregation]]
=== Geo Centroid Aggregation
-A metric aggregation that computes the weighted centroid from all coordinate values for a <<geo-point>> field.
+A metric aggregation that computes the weighted https://en.wikipedia.org/wiki/Centroid[centroid] from all coordinate values for a <<geo-point>> field.
Example:
