note restriction that only on <description> element is allowed in a

context

doc/ZConfig/zconfig.tex

@@ -386,6 +386,8 @@ The following elements are used to describe a schema:
   Descriptive text explaining the purpose the container of the
   \element{description} element.  Most other elements can contain
   a \element{description} element as their first child.
+  At most one \element{description} element may appear in a given
+  context.
 
   \begin{attributedesc}{format}{NMTOKEN}
     Optional attribute that can be added to indicate what conventions