doc: Add another stall-warning root cause in stallwarn.rst

This commit adds a bullet item noting that both deficiencies and surpluses
of calls to rcu_*_enter() and rcu_*_exit() can result in RCU CPU stall
warnings.

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

Documentation/RCU/stallwarn.rst

@@ -96,6 +96,16 @@ warnings:
 	the ``rcu_.*timer wakeup didn't happen for`` console-log message,
 	which will include additional debugging information.
 
+-	A low-level kernel issue that either fails to invoke one of the
+	variants of rcu_user_enter(), rcu_user_exit(), rcu_idle_enter(),
+	rcu_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one
+	hand, or that invokes one of them too many times on the other.
+	Historically, the most frequent issue has been an omission
+	of either irq_enter() or irq_exit(), which in turn invoke
+	rcu_irq_enter() or rcu_irq_exit(), respectively.  Building your
+	kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
+	of issues, which sometimes arise in architecture-specific code.
+
 -	A bug in the RCU implementation.
 
 -	A hardware failure.  This is quite unlikely, but has occurred