envelope doco fix

git-svn-id: http://svn.osgeo.org/postgis/trunk@2771 b70326c6-7e19-0410-871a-916f4a2858ee

diff --git a/doc/postgis.xml b/doc/postgis.xml
index fb6781753b0d7c7b4b853994bb6e9557e6f012d7..fcce7ee73391cb2cd5836008dc693441dfe26872 100644 (file)
--- a/doc/postgis.xml
+++ b/doc/postgis.xml
@@ -3340,8 +3340,10 @@ dimension
             <term>ST_Envelope(geometry)</term>
 
             <listitem>
-              <para>Returns a POLYGON representing the bounding box of the
-              geometry.</para>
+              <para>Returns a vald geometry (POINT, LINESTRING or POLYGON) 
+              representing the bounding box of the geometry. Degenerate
+              cases (vertical lines, point) will return a geometry of
+              lower dimension than POLYGON.</para>
 
               <para>OGC SPEC s2.1.1.1 - The minimum bounding box for this
               Geometry, returned as a Geometry. The polygon is defined by the