rcu: Document concurrent quiescent state reporting for offline CPUs

The synchronization of CPU offlining with GP initialization is confusing
to put it mildly (rightfully so as the issue it deals with is complex).
Recent discussions brought up a question -- what prevents the
rcu_implicit_dyntick_qs() from warning about QS reports for offline
CPUs (missing QS reports for offline CPUs causing indefinite hangs).

QS reporting for now-offline CPUs should only happen from:
- gp_init()
- rcutree_cpu_report_dead()

Add some documentation on this and refer to it from comments in the code
explaining how QS reporting is not missed when these functions are
concurrently running.

I referred heavily to this post [1] about the need for the ofl_lock.
[1] https://lore.kernel.org/all/20180924164443.GF4222@linux.ibm.com/

[ Applied paulmck feedback on moving documentation to Requirements.rst ]

Link: https://lore.kernel.org/all/01b4d228-9416-43f8-a62e-124b92e8741a@paulmck-laptop/


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