docs: Update RCU's hotplug requirements with a bit about design

The rcu_barrier() section of the "Hotplug CPU" section discusses
deadlocks, however the description of deadlocks other than those involving
rcu_barrier() is rather incomplete.

This commit therefore continues the section by describing how RCU's
design handles CPU hotplug in a deadlock-free way.

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