[headers, diff.mods.to.headers] Re-add main index entries for C headers that do not exist in C++

Author: tkoeppe

source/compatibility.tex

@@ -1926,9 +1926,9 @@
 
 \pnum
 There are no \Cpp{} headers for the C headers
-\tcode{<stdatomic.h>},
-\tcode{<stdnoreturn.h>},
-and \tcode{<threads.h>},
+\tcode{<stdatomic.h>}\indextext{\idxhdr{stdatomic.h}!absence thereof},
+\tcode{<stdnoreturn.h>}\indextext{\idxhdr{stdnoreturn.h}!absence thereof},
+and \tcode{<threads.h>}\indextext{\idxhdr{threads.h}!absence thereof},
 nor are the C headers themselves part of \Cpp{}.
 
 \pnum

source/lib-intro.tex

@@ -1129,9 +1129,9 @@
 additional headers shown in Table~\ref{tab:cpp.c.headers}.%
 \footnote{It is intentional that there is no \Cpp{} header
 for any of these C headers:
-\tcode{<stdatomic.h>},
-\tcode{<stdnoreturn.h>},
-\tcode{<threads.h>}.}
+\tcode{<stdatomic.h>}\indextext{\idxhdr{stdatomic.h}!absence thereof},
+\tcode{<stdnoreturn.h>}\indextext{\idxhdr{stdnoreturn.h}!absence thereof},
+\tcode{<threads.h>}\indextext{\idxhdr{threads.h}!absence thereof}.}
 
 \begin{floattable}{\Cpp{} headers for C library facilities}{tab:cpp.c.headers}
 {lllll}