linux-phytec-fsl

documentation: Document illegality of call_rcu() from offline CPUs

There is already a blanket statement about no member of RCU's API
being legal from an offline CPU, but add an explicit note where it
states that it is illegal to invoke call_rcu() from an NMI handler.

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