Document what \int_to_roman:n does beyond 5000 [ci skip] (fixes #540)

l3kernel/l3int.dtx

@@ -698,9 +698,13 @@
 %     \cs{int_to_roman:n} \Arg{integer expression}
 %   \end{syntax}
 %   Places the value of the \meta{integer expression} in the input
-%   stream as Roman numerals, either lower case (\cs{int_to_roman:n})
-%   or upper case (\cs{int_to_Roman:n}). The Roman numerals are letters
-%   with category code $11$ (letter).
+%   stream as Roman numerals, either lower case (\cs{int_to_roman:n}) or
+%   upper case (\cs{int_to_Roman:n}).  If the value is negative or zero,
+%   the output is empty.  The Roman numerals are letters with category
+%   code $11$ (letter).  The letters used are |mdclxvi|, repeated as
+%   needed: the notation with bars (such as $\bar{\mbox{v}}$ for $5000$)
+%   is \emph{not} used.  For instance \cs{int_to_roman:n} |{| 8249 |}|
+%   expands to |mmmmmmmmccxlix|.
 % \end{function}
 %
 % \section{Converting from other formats to integers}