explain some terminology that's helpful in talking about section type

extension

doc/ZConfig/zconfig.tex

@@ -437,7 +437,9 @@ The following elements are used to describe a schema:
     \emph{not} automatically implement any abstract section type
     implemented by the named section type.  If omitted, this section
     is defined with only the keys and sections contained within the
-    \element{sectiontype} element.
+    \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.
   \end{attributedesc}
 
   \begin{attributedesc}{implements}{\datatype{basic-key}}