Fixed #24144 -- Documented that GEOSGeometry.envelope can return a point

Thanks Manel Clos for the report.

docs/ref/contrib/gis/geos.txt

@@ -480,7 +480,8 @@ the geometry.
 .. attribute:: GEOSGeometry.envelope
 
 Returns a :class:`Polygon` that represents the bounding envelope of
-this geometry.
+this geometry. Note that it can also return a :class:`Point` if the input
+geometry is a point.
 
 .. attribute:: GEOSGeometry.point_on_surface