One of lclaudio's systems was seeing RCU CPU stall warnings from idle.
These turned out to be caused by a bug that stopped scheduling-clock
tick interrupts from being sent to a given CPU for several hundred seconds.
This commit therefore updates the documentation to call this out as a
possible cause for RCU CPU stall warnings.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>