Documentation/litmus-tests: Add note on herd7 7.56 in atomic litmus test

herdtools 7.56 has enhanced herd7's C parser so that the "(void)expr"
construct in Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus is
accepted.

This is independent of LKMM's cat model, so mention the required
version in the header of the litmus test and its entry in README.

CC: Boqun Feng <boqun.feng@gmail.com>
Reported-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/litmus-tests/README

@@ -19,6 +19,7 @@ Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus
 
 Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus
     Test that atomic_set() cannot break the atomicity of atomic RMWs.
+    NOTE: Require herd7 7.56 or later which supports "(void)expr".
 
 
 RCU (/rcu directory)

Documentation/litmus-tests/atomic/Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus

@@ -4,6 +4,7 @@ C Atomic-RMW-ops-are-atomic-WRT-atomic_set
  * Result: Never
  *
  * Test that atomic_set() cannot break the atomicity of atomic RMWs.
+ * NOTE: This requires herd7 7.56 or later which supports "(void)expr".
  *)
 
 {