doc: Document <stdatomic.h>

[Bruno Haible]

There's one ISO C header file that we don't have in the Gnulib documentation
so far: <stdatomic.h>. Documented in
  - ISO C 11 § 7.17,
  - https://en.cppreference.com/w/c/atomic

This patch fixes that.


2023-06-09  Bruno Haible  <bruno@clisp.org>

        doc: Document <stdatomic.h>.
        * doc/posix-headers/stdatomic.texi: New file.
        * doc/gnulib.texi (Header File Substitutes): Include it.

diff --git a/doc/gnulib.texi b/doc/gnulib.texi
index 0f91de5a39..4b47749fe6 100644
--- a/doc/gnulib.texi
+++ b/doc/gnulib.texi
@@ -1076,6 +1076,7 @@
 * spawn.h::
 * stdalign.h::
 * stdarg.h::
+* stdatomic.h::
 * stdbool.h::
 * stdckdint.h::
 * stddef.h::
@@ -1167,6 +1168,7 @@
 @include posix-headers/spawn.texi
 @include posix-headers/stdalign.texi
 @include posix-headers/stdarg.texi
+@include posix-headers/stdatomic.texi
 @include posix-headers/stdbool.texi
 @include posix-headers/stdckdint.texi
 @include posix-headers/stddef.texi
diff --git a/doc/posix-headers/stdatomic.texi b/doc/posix-headers/stdatomic.texi
new file mode 100644
index 0000000000..42466b7e43
--- /dev/null
+++ b/doc/posix-headers/stdatomic.texi
@@ -0,0 +1,20 @@
+@node stdatomic.h
+@section @file{stdatomic.h}
+
+POSIX specification:@* Not in POSIX yet, but we expect it will be.
+ISO C11 (latest free draft
+@url{http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf})
+section 7.17.
+
+Gnulib module: ---
+
+Portability problems fixed by Gnulib:
+@itemize
+@end itemize
+
+Portability problems not fixed by Gnulib:
+@itemize
+@item
+This header file is missing on many platforms:
+glibc with GCC 4.8.x or clang 2.8, FreeBSD 6.4, NetBSD 7.0, OpenBSD 6.5, 
OpenBSD 6.9 with gcc, Minix 3.3.0, AIX 7.2 with xlc or xlclang, HP-UX 11.31, 
IRIX 6.5, Solaris 11.3 with Oracle Studio 12.4 (Sun C 5.13), Cygwin 1.5.19, 
mingw, MSVC 14.
+@end itemize
-- 
2.34.1