The current documentation did not help someone grepping for SRCU to
learn that disabling preemption is not a replacement for srcu_read_lock(),
so upgrade the documentation to bring this out, not just for SRCU,
but also for RCU-bh.  Also document the fact that SRCU readers are
respected on CPUs executing in user mode, idle CPUs, and even on
offline CPUs.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>