Fixed #26266 -- Output the primary key in the GeoJSON serializer properties

Thanks Tim Graham for the review.

docs/ref/contrib/gis/serializers.txt

@@ -71,3 +71,11 @@ Would output::
         }
       ]
     }
+
+When the ``fields`` parameter is not specified, the ``geojson`` serializer adds
+a ``pk`` key to the ``properties`` dictionary with the primary key of the
+object as the value.
+
+.. versionchanged:: 1.10
+
+    The ``pk`` key was added to the ``properties`` dictionary.