rcu: Document separation of rcu_state and rnp's gp_seq

The details of this are subtle and was discussed recently. Add a
quick-quiz about this and refer to it from the code, for more clarity.

Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>