[defns.undefined] Fix the note on UB in constant evaluation

Currently, the `assmue` and `noreturn` attributes are allowed to bring undefined behavior into constant evaluation. There might be more such standard attributes in the future.

Author: frederick-vs-ja

source/intro.tex

@@ -256,7 +256,8 @@
 Implementations are allowed, but not required,
 to diagnose it\iref{intro.compliance.general}.
 Evaluation of a constant expression\iref{expr.const}
-never exhibits behavior specified as erroneous in \ref{intro} through \ref{cpp}.
+never exhibits behavior specified as erroneous in \ref{intro} through \ref{cpp}
+with standard attributes\iref{dcl.attr} ignored.
 \end{defnote}
 
 \definition{expression-equivalent}{defns.expression.equivalent}