rcu: Add comment to rcu_do_batch() identifying rcuoc code path

This commit adds a comment to help explain why the "else" clause of the
in_serving_softirq() "if" statement does not need to enforce a time limit.
The reason is that this "else" clause handles rcuoc kthreads that do not
block handlers for other softirq vectors.

Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>