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>