docs: pm: Update runtime PM documentation and release notes

Add new configuration options for runtime PM to the release notes,
including stack size, priority, and system work queue usage. Update
the runtime PM documentation to explain the implications of using the
system work queue and disabling asynchronous operations. Include a new
version of the sequence diagram for asynchronous operations.

Signed-off-by: Flavio Ceolin <flavio@hubblenetwork.com>

Author: ceolin

doc/releases/release-notes-4.2.rst

@@ -139,6 +139,14 @@ New APIs and options
 
     * :kconfig:option:`CONFIG_MQTT_VERSION_5_0`
 
+* Power management
+
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_WQ_STACK_SIZE`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_WQ_PRIO`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_WQ_INIT_PRIO`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_USE_SYSTEM_WQ`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_ASYNC`
+
 * Stepper
 
   * :c:func:`stepper_stop()`

doc/services/pm/device_runtime.rst

@@ -107,9 +107,24 @@ be a problem if the operation is fast, e.g. a register toggle. However, the
 situation will not be the same if suspension involves sending packets through a
 slow bus. For this reason the device drivers can also make use of the
 :c:func:`pm_device_runtime_put_async` function. This function will schedule
-the suspend operation, again, if device is no longer used. The suspension will
-then be carried out when the system work queue gets the chance to run. The
-sequence diagram shown below illustrates this scenario.
+the suspend operation, again, if device is no longer used.
+
+By default, the operation is offloaded to the PM subsystem’s work
+queue. However, applications can configure the runtime PM to use the
+system work queue instead selecting
+:kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_USE_SYSTEM_WQ`.  This choice
+comes with a caveat: when using the system work queue, device drivers
+must not perform any blocking operations during suspend, as this could
+stall the system work queue. If blocking behavior is needed—for
+example, accessing a slow peripheral or waiting on a bus
+transaction—the PM subsystem work queue must be used to avoid
+impacting system responsiveness.
+
+For targets with constrained resources that do not need asynchronous
+operations, this functionality can be disabled altogether by
+de-selecting :kconfig:option:`PM_DEVICE_RUNTIME_ASYNC`, reducing
+memory usage and system complexity.
+
 
 .. figure:: images/devr-async-ops.svg