Add cStringIO to the list of modules in the chapter.

Doc/lib/libstrings.tex

@@ -24,4 +24,7 @@ manipulation operations.  Here's an overview:
 \item[StringIO]
 --- Read and write strings as if they were files.
 
+\item[cStringIO]
+--- Faster version of \module{StringIO}, but not subclassable.
+
 \end{description}

Doc/libstrings.tex

@@ -24,4 +24,7 @@ manipulation operations.  Here's an overview:
 \item[StringIO]
 --- Read and write strings as if they were files.
 
+\item[cStringIO]
+--- Faster version of \module{StringIO}, but not subclassable.
+
 \end{description}