[diff.cpp17.input.output] Add ill-formed comment and a still well-formed use.

Author: Dawn Perchik

source/compatibility.tex

@@ -2029,7 +2029,9 @@
 Valid \CppXVII{} code may fail to compile in this International Standard:
 \begin{codeblock}
 auto p = new char[100];
-std::cin >> std::setw(20) >> p;
+char q[100];
+std::cin >> std::setw(20) >> p;        // ill-formed; previously well-formed
+std::cin >> std::setw(20) >> q;        // OK
 \end{codeblock}
 
 \rSec2[diff.cpp17.depr]{\ref{depr}: compatibility features}