rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU

The cond_resched() function reports an RCU quiescent state only in
non-preemptible TREE RCU implementation.  This commit therefore adds a
comment explaining why cond_resched() does nothing in preemptible kernels.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>