documentation: Emphasize the call_rcu() is illegal from idle

Although call_rcu()'s fastpath works just fine on an idle CPU,
some branches of the slowpath invoke the scheduler, which uses
RCU.  Therefore, this commit emphasizes the fact that call_rcu()
must not be invoked from an idle CPU.

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

Documentation/RCU/Design/Requirements/Requirements.html

@@ -1650,7 +1650,7 @@ situations where neither <tt>synchronize_rcu()</tt> nor
 including within preempt-disable code, <tt>local_bh_disable()</tt> code,
 interrupt-disable code, and interrupt handlers.
 However, even <tt>call_rcu()</tt> is illegal within NMI handlers
-and from offline CPUs.
+and from idle and offline CPUs.
 The callback function (<tt>remove_gp_cb()</tt> in this case) will be
 executed within softirq (software interrupt) environment within the
 Linux kernel,