CWG2538 Can standard attributes be syntactically ignored?

Also fixes NB GB 055 (C++23 CD).

Author: burblebee

source/declarations.tex

@@ -8408,9 +8408,15 @@
 For an \grammarterm{attribute-token}
 (including an \grammarterm{attribute-scoped-token})
 not specified in this document, the
-behavior is \impldef{behavior of non-standard attributes}.
-Any \grammarterm{attribute-token} that is not recognized by the implementation
+behavior is \impldef{behavior of non-standard attributes};
+any such \grammarterm{attribute-token} that is not recognized by the implementation
 is ignored.
+\begin{note}
+A program is ill-formed if it contains an \grammarterm{attribute}
+specified in \ref{dcl.attr} that violates
+the rules specifying to which entity or statement the attribute may apply or
+the syntax rules for the attribute's \grammarterm{attribute-argument-clause}, if any.
+\end{note}
 An \grammarterm{attribute-token} is reserved for future standardization if
 \begin{itemize}
 \item it is not an \grammarterm{attribute-scoped-token} and