rcuscale: Add kfree_by_call_rcu and kfree_mult to documentation

This commit adds the kfree_by_call_rcu and kfree_mult rcuscale module
parameters to kernel-parameters.txt.  While in the area, it updates
to rcuscale.scale_type.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>