rcu-tasks: Document that RCU Tasks Trace grace periods now imply RCU grace periods

rcu-tasks: Document that RCU Tasks Trace grace periods now imply RCU grace periods

Now that RCU Tasks Trace is implemented in terms of SRCU-fast, the fact
that each SRCU-fast grace period implies at least two RCU grace periods
in turn means that each RCU Tasks Trace grace period implies at least
two grace periods.  This commit therefore updates the documentation
accordingly.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>