add remarck on mixed struct+unstruct vol meshes with old iso 3d algo

doc/texinfo/gmsh.texi

 \input texinfo.tex @c -*-texinfo-*-
-@c $Id: gmsh.texi,v 1.205 2006-03-10 02:30:43 geuzaine Exp $
+@c $Id: gmsh.texi,v 1.206 2006-03-22 12:37:46 geuzaine Exp $
 @c
 @c Copyright (C) 1997-2006 C. Geuzaine, J.-F. Remacle
 @c
@@ -368,10 +368,15 @@ length field, which can be uniform, specified by characteristic lengths
 associated with elementary geometrical entities, or associated with another
 mesh (the background mesh).
 
-For each meshing step, all structured mesh directives are executed first,
-and serve as additional constraints for the unstructured parts. The
-implemented Delaunay algorithm is subdivided in the following five steps for
-surface/volume discretization:
+For each meshing step, all structured mesh directives are executed
+first, and serve as additional constraints for the unstructured parts
+@footnote{Note that mixing structured volume grids with unstructured
+volume grids generated with the default 3D isotropic Delaunay algorithm
+can result, in certain cases, to non-conform surface meshes on their
+shared boudary. If this happens, you may consider using the Netgen
+algorithm for the unstructured part.}. The implemented Delaunay
+algorithm is subdivided in the following five steps for surface/volume
+discretization:
 
 @enumerate
 @item