Add DPC's text re. Unicode/upTeX

base/doc/ltnews31.tex

@@ -629,7 +629,9 @@ \section{Primitive requirements}
 \endgroup
 Note that it has become standard practice to check for Unicode-aware engines
 with the existence of the \cs{Umathcode} primitive. As such, this is already
-a requirement: engines lacking these primitives cannot access Unicode features.
+a requirement: engines lacking these primitives cannot access Unicode features
+of the \LaTeXe{} kernel or of \pkg{expl3}. Note that up\TeX{} has facilities for
+handling Unicode but is not classed as a Unicode engine by the base LaTeX code.
 
 \begin{thebibliography}{9}