Describe the search up the containment chain for substitution values.

doc/ZConfig/zconfig.tex

@@ -547,8 +547,13 @@ Notes:
 \end{description}
 
 In each case, \var{name} is a non-empty sequence of alphanumeric and
-underscore characters not starting with a digit.  If there is not
-a replacement for \var{name}, it is replaced with an empty string.
+underscore characters not starting with a digit.  If there is not a
+replacement for \var{name}, these functions search up the containment
+chain for a suitable replacement.  The containment chain is found by
+looking for an attribute \member{container} on the \var{mapping}
+object; the value of that attribute should be another mapping or
+\code{None}.  If this search does not yield a value, an empty string
+is used.
 
 For these functions, the \var{mapping} argument can be a \class{dict},
 or any type that supports the \method{get()} method of the mapping