doc: Timer problems can cause RCU CPU stall warnings

Over the past few years, there have been several cases where timekeeping
bugs have caused RCU CPU stall warnings, particularly during hardware
bringup.  This commit therefore adds such bugs to the list of things
that can result in RCU CPU stall warnings.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/stallwarn.rst

@@ -87,6 +87,13 @@ warnings:
 	problem really has happened, and seems to be most likely to
 	result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
 
+-	A hardware or software issue that prevents time-based wakeups
+	from occurring.  These issues can range from misconfigured or
+	buggy timer hardware through bugs in the interrupt or exception
+	path (whether hardware, firmware, or software) through bugs
+	in Linux's timer subsystem through bugs in the scheduler, and,
+	yes, even including bugs in RCU itself.
+
 -	A bug in the RCU implementation.
 
 -	A hardware failure.  This is quite unlikely, but has occurred