Added ref to StringIO docs.

Doc/lib/libstrings.tex

@@ -17,4 +17,7 @@ manipulation operations.  Here's an overview:
 \item[struct]
 --- Interpret strings as packed binary data.
 
+\item[StringIO]
+--- Read and write strings as if they were files.
+
 \end{description}

Doc/libstrings.tex

@@ -17,4 +17,7 @@ manipulation operations.  Here's an overview:
 \item[struct]
 --- Interpret strings as packed binary data.
 
+\item[StringIO]
+--- Read and write strings as if they were files.
+
 \end{description}