Comment on the status of ATOMIC_VAR_INIT() in the POSIX test.

See "FUTURE DIRECTIONS" in https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdatomic.h.html, but note also that source compatibility -- and the harmlessness of a no-op macro -- suggest we should keep this forever, regardless.

Change-Id: I8c3b4e57ca420596423019fce05f6f30b31f7f33
diff --git a/tests/headers/posix/stdatomic_h.c b/tests/headers/posix/stdatomic_h.c
index 9f46e6a..270faae 100644
--- a/tests/headers/posix/stdatomic_h.c
+++ b/tests/headers/posix/stdatomic_h.c
@@ -66,6 +66,9 @@
   MACRO(ATOMIC_POINTER_LOCK_FREE);
 
   atomic_flag f = ATOMIC_FLAG_INIT;
+
+  // ATOMIC_VAR_INIT() has been removed from C23,
+  // but not from POSIX 2024.
   atomic_int i = ATOMIC_VAR_INIT(123);
 
   i = kill_dependency(i);