diff --git a/docs/ref/contrib/gis/functions.txt b/docs/ref/contrib/gis/functions.txt
index 06fe57fe996..490a1ef13dd 100644
--- a/docs/ref/contrib/gis/functions.txt
+++ b/docs/ref/contrib/gis/functions.txt
@@ -55,7 +55,9 @@ AsGeoJSON
 *Availability*: PostGIS, SpatiaLite
 
 Accepts a single geographic field or expression and returns a `GeoJSON
-<http://geojson.org/>`_ representation of the geometry.
+<http://geojson.org/>`_ representation of the geometry. Note that the result is
+not a complete GeoJSON structure but only the ``geometry`` key content of a
+GeoJSON structure. See also :doc:`/ref/contrib/gis/serializers`.
 
 Example::
 
diff --git a/docs/ref/contrib/gis/geos.txt b/docs/ref/contrib/gis/geos.txt
index d2e4bafe006..47f815d4cbd 100644
--- a/docs/ref/contrib/gis/geos.txt
+++ b/docs/ref/contrib/gis/geos.txt
@@ -282,7 +282,9 @@ that are a part of this geometry.
 
 .. attribute:: GEOSGeometry.json
 
-Returns the GeoJSON representation of the geometry.
+Returns the GeoJSON representation of the geometry. Note that the result is not
+a complete GeoJSON structure but only the ``geometry`` key content of a
+GeoJSON structure. See also :doc:`/ref/contrib/gis/serializers`.
 
 .. note::