[diff.cpp23.library] Entry for new headers in C++26 (#6648)

Author: AlisdairM

source/compatibility.tex

@@ -31,6 +31,21 @@
 bool b = ne({2,3}, {1,2,3});    // unspecified result; previously \tcode{false}
 \end{codeblock}
 
+\rSec2[diff.cpp23.library]{\ref{library}: library introduction}
+
+\diffref{headers}
+\change
+New headers.
+\rationale
+New functionality.
+\effect
+The following \Cpp{} headers are new:
+\libheaderrefx{hazard_pointer}{hazard.pointer.syn},
+\libheaderref{rcu}, and
+\libheaderrefx{text_encoding}{text.encoding.syn}.
+Valid \CppXXIII{} code that \tcode{\#include}{s} headers with these names may be
+invalid in this revision of \Cpp{}.
+
 \rSec2[diff.cpp23.strings]{\ref{strings}: strings library}
 
 \diffref{string.conversions}