rcu: Advise most users not to enable RCU user mode

Discourage distros from enabling CONFIG_RCU_USER_QS
because it brings overhead for no benefits yet.

It's not a useful feature on its own until we can
fully run an adaptive tickless kernel.

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