added description of StringTypes object

Doc/lib/libtypes.tex

@@ -155,3 +155,8 @@ The type of frame objects such as found in \code{tb.tb_frame} if
 The type of buffer objects created by the
 \function{buffer()}\bifuncindex{buffer} function.
 \end{datadesc}
+
+\begin{datadesc}{StringTypes}
+A list containing StringType and UnicodeTypes used to facilitate easier
+checking for any string object, e.g. \code{s in types.StringTypes}.
+\end{datadesc}