allow "keytype" to be overridden in derived section types

doc/ZConfig/zconfig.tex

@@ -442,9 +442,9 @@ The following elements are used to describe a schema:
     is defined with only the keys and sections contained within the
     \element{sectiontype} element.  The new section type is called a
     \emph{derived} section type, and the type named by this attribute
-    is called the \emph{base} type.  The value for the
-    \attribute{datatype} attribute is acquired from the base type if
-    not specified.
+    is called the \emph{base} type.  Values for the
+    \attribute{datatype} and \attribute{keytype} attributes are
+    acquired from the base type if not specified.
   \end{attributedesc}
 
   \begin{attributedesc}{implements}{\datatype{basic-key}}
@@ -461,7 +461,10 @@ The following elements are used to describe a schema:
     the \datatype{identifier} and \datatype{ipaddr-or-hostname}
     types.  If the value is a \datatype{dotted-name} that begins
     with a period, the value of \attribute{prefix} will be pre-pended,
-    if set.  The default value is \datatype{basic-key}.
+    if set.  The default value is \datatype{basic-key}.  If
+    \attribute{keytype} is omitted and \attribute{extends} is used,
+    the \attribute{keytype} from the section type identified by the
+    \attribute{extends} attribute is used.
   \end{attributedesc}
 
   \begin{attributedesc}{name}{\datatype{basic-key}}