rcu: Further comment and explain the state space of GP sequences

The state space of the GP sequence number isn't documented and the
definitions of its special values are scattered.  This commit therefore
gathers some common knowledge near the grace-period sequence-number
definitions.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-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>