doc: Make whatisRCU.rst note that spinlocks are RCU readers

In kernels built with CONFIG_PREEMPT_RT=n, spinlock critical sections
are RCU readers because they disable preemption.  However, they are also
RCU readers in CONFIG_PREEMPT_RT=y because in that case the locking
primitives contain rcu_read_lock() and rcu_read_unlock().  Therefore,
upgrade whatisRCU.rst to document this non-obvious case.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>