[lex.name] Add cross-reference to Annex E

Fixes ISO/CS 030 (C++23 DIS).

Author: jensmaurer

source/lex.tex

@@ -840,6 +840,10 @@
 Identifiers are case-sensitive.
 \end{note}
 \begin{note}
+\ref{uaxid} compares the requirements of \UAX{31} of the Unicode Standard
+with the \Cpp{} rules for identifiers.
+\end{note}
+\begin{note}
 In translation phase 4,
 \grammarterm{identifier} also includes
 those \grammarterm{preprocessing-token}s\iref{lex.pptoken}