Document the "printable" and "punctuation" constants added to the string

module.

Doc/lib/libstring.tex

@@ -37,6 +37,17 @@ The constants defined in this module are are:
   The string \code{'01234567'}.
 \end{datadesc}
 
+\begin{datadesc}{punctuation}
+  String of \ASCII{} characters which are considered punctuation
+  characters in the \samp{C} locale.
+\end{datadesc}
+
+\begin{datadesc}{printable}
+  String of characters which are considered printable.  This is a
+  combination of \constant{digits}, \constant{letters},
+  \constant{punctuation}, and \constant{whitespace}.
+\end{datadesc}
+
 \begin{datadesc}{uppercase}
   A string containing all the characters that are considered uppercase
   letters.  On most systems this is the string