rcu: Fix RCU_FANOUT help message

Commit cf244dc0 added a fourth level to the TREE_RCU hierarchy,
but the RCU_FANOUT help message still said "cube root".  This commit
fixes this to "fourth root" and also emphasizes that production
systems are well-served by the default.  (Stress-testing RCU itself
uses small RCU_FANOUT values in order to test large-system code paths
on small(er) systems.)

Located-by: John Kacur <jkacur@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>