Add a note about when the "%r" formatting code was added.

Doc/lib/libstdtypes.tex

@@ -816,6 +816,7 @@ The conversion types are:
 
 % XXX Examples?
 
+(The \code{\%r} conversion was added in Python 2.0.)
 
 Since Python strings have an explicit length, \code{\%s} conversions
 do not assume that \code{'\e0'} is the end of the string.