PM / doc: Update device documentation for devices in IRQ-safe PM domains

Update documentation to reflect the changes made to support IRQ safe PM
domains.

Suggested-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Lina Iyer <lina.iyer@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/power/devices.txt

@@ -607,7 +607,9 @@ individually.  Instead, a set of devices sharing a power resource can be put
 into a low-power state together at the same time by turning off the shared
 power resource.  Of course, they also need to be put into the full-power state
 together, by turning the shared power resource on.  A set of devices with this
-property is often referred to as a power domain.
+property is often referred to as a power domain. A power domain may also be
+nested inside another power domain. The nested domain is referred to as the
+sub-domain of the parent domain.
 
 Support for power domains is provided through the pm_domain field of struct
 device.  This field is a pointer to an object of type struct dev_pm_domain,
@@ -629,6 +631,16 @@ support for power domains into subsystem-level callbacks, for example by
 modifying the platform bus type.  Other platforms need not implement it or take
 it into account in any way.
 
+Devices may be defined as IRQ-safe which indicates to the PM core that their
+runtime PM callbacks may be invoked with disabled interrupts (see
+Documentation/power/runtime_pm.txt for more information).  If an IRQ-safe
+device belongs to a PM domain, the runtime PM of the domain will be
+disallowed, unless the domain itself is defined as IRQ-safe. However, it
+makes sense to define a PM domain as IRQ-safe only if all the devices in it
+are IRQ-safe. Moreover, if an IRQ-safe domain has a parent domain, the runtime
+PM of the parent is only allowed if the parent itself is IRQ-safe too with the
+additional restriction that all child domains of an IRQ-safe parent must also
+be IRQ-safe.
 
 Device Low Power (suspend) States
 ---------------------------------