Add some index entries to make it easier to find information on

%-formatting of strings.

Doc/lib/libstdtypes.tex

@@ -586,6 +586,11 @@ Return a copy of the string converted to uppercase.
 
 \subsubsection{String Formatting Operations \label{typesseq-strings}}
 
+\index{formatting, string}
+\index{string!formatting}
+\index{printf-style formatting}
+\index{sprintf-style formatting}
+
 String objects have one unique built-in operation: the \code{\%}
 operator (modulo) with a string left argument interprets this string
 as a C \cfunction{sprintf()} format string to be applied to the