PM / Documentation: Update docs about suspend and CPU hotplug

Update the documentation about the interaction between the suspend (S3) call
path and the CPU hotplug infrastructure.
This patch focusses only on the activities of the freezer, cpu hotplug and
the notifications involved. It outlines how regular CPU hotplug differs from
the way it is invoked during suspend and also tries to explain the locking
involved. In addition to that, it discusses the issue of microcode update
during CPU hotplug operations.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

Documentation/power/00-INDEX

@@ -26,6 +26,8 @@ s2ram.txt
 	- How to get suspend to ram working (and debug it when it isn't)
 states.txt
 	- System power management states
+suspend-and-cpuhotplug.txt
+	- Explains the interaction between Suspend-to-RAM (S3) and CPU hotplug
 swsusp-and-swap-files.txt
 	- Using swap files with software suspend (to disk)
 swsusp-dmcrypt.txt