[diff.cpp20.library] Add missing new headers for C++23

Author: AlisdairM

source/compatibility.tex

@@ -202,10 +202,12 @@
 \libheaderrefx{flat_map}{flat.map.syn},
 \libheaderrefx{flat_set}{flat.set.syn},
 \libheaderref{generator},
+\libheaderref{mdspan},
 \libheaderref{print},
 \libheaderref{spanstream},
-\libheaderref{stacktrace}, and
-\libheaderref{stdatomic.h}.
+\libheaderref{stacktrace},
+\libheaderref{stdatomic.h}, and
+\libheaderref{stdfloat}.
 Valid \CppXX{} code that \tcode{\#include}{s} headers with these names may be
 invalid in this revision of \Cpp{}.