[diff.cpp20.library] Add subclause, highlighting new headers

Author: jensmaurer

source/compatibility.tex

@@ -55,6 +55,22 @@
                         // now equivalent to \tcode{arr.operator[](1, 2)} or ill-formed
 \end{codeblock}
 
+\rSec2[diff.cpp20.library]{\ref{library}: library introduction}
+
+\diffref{headers}
+\change
+New headers.
+\rationale
+New functionality.
+\effect
+The following \Cpp{} headers are new:
+\libheaderref{expected},
+\libheaderref{stdatomic.h},
+\libheaderref{spanstream}, and
+\libheaderref{stacktrace}.
+Valid \CppXX{} code that \tcode{\#include}{s} headers with these names may be
+invalid in this revision of \Cpp{}.
+
 \rSec2[diff.cpp20.utilities]{\ref{utilities}: general utilities library}
 
 \diffref{format}