forgot to mark use of StringType and UnicodeType in the text.

Doc/lib/libtypes.tex

@@ -157,6 +157,7 @@ The type of buffer objects created by the
 \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}.
+A list containing \var{StringType} and \var{UnicodeType} used to
+facilitate easier checking for any string object, e.g. \code{s in
+types.StringTypes}.
 \end{datadesc}