explicit warning about half circles

doc/FAQ

-GMSH FREQUENTLY ASKED QUESTIONS ($Date: 2002-01-21 19:43:38 $)
+GMSH FREQUENTLY ASKED QUESTIONS ($Date: 2002-03-04 16:45:21 $)
 
 1. GENERAL
 ==========
@@ -103,7 +103,10 @@ Send us your geometry, and we will investigate. Please keep the
 following in mind though: 2D (surface) meshes are generated by
 projecting a 2D mesh in the mean plane of the surface. This gives nice
 results only if the surface curvature is small enough. Otherwise you
-_have_ to cut the surface in pieces.
+_have_ to cut the surface in pieces. For example, using use half
+circles to define a cylinder will fail with the unstructured algorithm
+(you should define arcs with angles smaller than 2*Pi, and thus
+define the cylinder with at least three patch surfaces).
 
 5.3) What should I do when the 3D unstructured algorithm fails?