tools/memory-model/Documentation: Put redefinition of rcu-fence into explanation.txt

This patch updates the Linux Kernel Memory Model's explanation.txt
file to incorporate the introduction of the rcu-order relation and
the redefinition of rcu-fence made by commit 15aa25cb


("tools/memory-model: Change definition of rcu-fence").

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>