update docs

Author: parkertimmins

docs/reference/elasticsearch/mapping-reference/geo-point.md

@@ -218,7 +218,9 @@ of official GA features.
 
 
 Synthetic source may sort `geo_point` fields (first by latitude and then
-longitude) and reduces them to their stored precision. For example:
+longitude) and reduces them to their stored precision. Additionally, unlike most
+types, arrays of `geo_point` fields will not store their source even when
+`synthetic_source_keep` is set to `arrays`. For example:
 
 $$$synthetic-source-geo-point-example$$$
 
@@ -229,7 +231,8 @@ PUT idx
     "index": {
       "mapping": {
         "source": {
-          "mode": "synthetic"
+          "mode": "synthetic",
+          "synthetic_source_keep": "arrays"
         }
       }
     }