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>