documentation: Fix relation between nohz_full and rcu_nocbs

If a CPU is specified in the nohz_full= kernel boot parameter to a
kernel built with CONFIG_NO_HZ_FULL=y, then that CPU's callbacks will
be offloaded, just as if that CPU had also been specified in the
rcu_nocbs= kernel boot parameter.  But the current documentation
states that the user must keep these two boot parameters manually
synchronized.  This commit therefore updates the documentation to
reflect reality.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>