diff --git a/capi/geos_c.h.in b/capi/geos_c.h.in
index 69ef1f8c52..1e24433759 100644
--- a/capi/geos_c.h.in
+++ b/capi/geos_c.h.in
@@ -3785,7 +3785,7 @@ static const unsigned GEOS_CLUSTER_NONE = 4294967295;
  * @param eps distance parameter for clustering
  * @param minPoints density parameter for clustering
  * @param numClusters the number of clusters formed
- * @return an array of cluster identifiers
+ * @return an array of cluster identifiers to free with GEOSFree()
  */
 extern unsigned GEOS_DLL* GEOSClusterDBSCAN(const GEOSGeometry* g, double eps, unsigned minPoints,
         unsigned* numClusters);
@@ -3798,7 +3798,7 @@ extern unsigned GEOS_DLL* GEOSClusterDBSCAN(const GEOSGeometry* g, double eps, u
  * @param g a collection of geometries to be clustered
  * @param d minimum distance between geometries in the same cluster
  * @param numClusters the number of clusters formed
- * @return an array of cluster identifiers
+ * @return an array of cluster identifiers to free with GEOSFree()
  */
 extern unsigned GEOS_DLL* GEOSClusterGeometryDistance(const GEOSGeometry* g, double d, unsigned* numClusters);
 
@@ -3809,7 +3809,7 @@ extern unsigned GEOS_DLL* GEOSClusterGeometryDistance(const GEOSGeometry* g, dou
  *
  * @param g a collection of geometries to be clustered
  * @param numClusters the number of clusters formed
- * @return an array of cluster identifiers
+ * @return an array of cluster identifiers to free with GEOSFree()
  */
 extern unsigned GEOS_DLL* GEOSClusterGeometryIntersects(const GEOSGeometry* g, unsigned* numClusters);
 
@@ -3821,7 +3821,7 @@ extern unsigned GEOS_DLL* GEOSClusterGeometryIntersects(const GEOSGeometry* g, u
  * @param g a collection of geometries to be clustered
  * @param d minimum envelope distance between geometries in the same cluster
  * @param numClusters the number of clusters formed
- * @return an array of cluster identifiers
+ * @return an array of cluster identifiers to free with GEOSFree()
  */
 extern unsigned GEOS_DLL* GEOSClusterEnvelopeDistance(const GEOSGeometry* g, double d, unsigned* numClusters);
 
@@ -3832,7 +3832,7 @@ extern unsigned GEOS_DLL* GEOSClusterEnvelopeDistance(const GEOSGeometry* g, dou
  *
  * @param g
  * @param numClusters the number of clusters formed
- * @return an array of cluster identifiers
+ * @return an array of cluster identifiers to free with GEOSFree()
  */
 extern unsigned GEOS_DLL* GEOSClusterEnvelopeIntersects(const GEOSGeometry* g, unsigned* numClusters);
 ///@}