doc: Update removal of RCU-bh/sched update machinery

doc: Update removal of RCU-bh/sched update machinery

The RCU-bh update API is now defined in terms of that of RCU-bh and
RCU-sched, so this commit updates the documentation accordingly.

In addition, although RCU-sched persists in !PREEMPT kernels, in
the PREEMPT case its update API is now defined in terms of that of
RCU-preempt, so this commit also updates the documentation accordingly.

While in the area, this commit removes the documentation for the
now-obsolete synchronize_rcu_mult() and clarifies the Tasks RCU
documentation.

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