Add a note about (u)pTeX and \codepoint_generate:nn

l3kernel/l3unicode.dtx

@@ -83,6 +83,13 @@
 %   $13$); for codepoints in the ASCII range, the given \meta{catcode}
 %   will be used. To allow the result of this function to be used
 %   inside a expansion context, the result is protected by \cs{exp_not:n}.
+%
+%   \begin{texnote}
+%     Users of (u)p\TeX{} note that these engines are treated as $8$-bit in
+%     this context. In particular, for up\TeX{}, irrespective of the
+%     \tn{kcatcode} of the \meta{codepoint}, any value outside the ASCII range
+%     will result in a series of active bytes being generated.
+%   \end{texnote}
 % \end{function}
 %
 % \begin{function}[EXP, added = 2022-10-09]