documentation: Call out slow consoles as cause of stall warnings

The Linux kernel outputs copious text during boot, and a slow serial
console can result in stall warnings, particularly when messages are
printed with interrupts disabled.  This commit adds this to the list
of causes of RCU CPU stall warning messages.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

Documentation/RCU/stallwarn.txt

@@ -205,6 +205,13 @@ o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
 	behavior, you might need to replace some of the cond_resched()
 	calls with calls to cond_resched_rcu_qs().
 
+o	Booting Linux using a console connection that is too slow to
+	keep up with the boot-time console-message rate.  For example,
+	a 115Kbaud serial console can be -way- too slow to keep up
+	with boot-time message rates, and will frequently result in
+	RCU CPU stall warning messages.  Especially if you have added
+	debug printk()s.
+
 o	Anything that prevents RCU's grace-period kthreads from running.
 	This can result in the "All QSes seen" console-log message.
 	This message will include information on when the kthread last