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>