[diff.cpp17.except] Add a note that there is no longer a way to write

code that is non-throwing in both C++20 and C++03.

Author: zygoloid

source/compatibility.tex

@@ -1966,6 +1966,13 @@
 for its exception specification will be rejected as ill-formed in this
 International Standard. It should simply be replaced with \tcode{noexcept} for no
 change of meaning since \CppXVII{}.
+\begin{note}
+There is no way to write a function declaration
+that is non-throwing in this International Standard
+and is also non-throwing in \CppIII{}
+except by using the preprocessor to generate
+a different token sequence in each case.
+\end{note}
 
 \rSec2[diff.cpp17.library]{\ref{library}: library introduction}