Add a reference to the material on string methods.

Doc/lib/libstring.tex

@@ -53,8 +53,11 @@ The constants defined in this module are are:
   is undefined.
 \end{datadesc}
 
-The functions defined in this module are:
 
+Many of the functions provided by this module are also defined as
+methods of string and Unicode objects; see ``String Methods'' (section
+\ref{string-methods}) for more information on those.
+The functions defined in this module are:
 
 \begin{funcdesc}{atof}{s}
   Convert a string to a floating point number.  The string must have