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 <[email protected]>

Author: ceolin

doc/releases/release-notes-4.2.rst

@@ -173,6 +173,16 @@ New APIs and options
 
     * :kconfig:option:`CONFIG_MQTT_VERSION_5_0`
 
+* Power management
+
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_USE_SYSTEM_WQ`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_USE_DEDICATED_WQ`
+    * :kconfig:option:`CONFIG_PM_DRIVER_NEEDS_DEDICATED_WQ`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_DEDICATED_WQ_STACK_SIZE`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_DEDICATED_WQ_PRIO`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_DEDICATED_WQ_INIT_PRIO`
+    * :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_ASYNC`
+
   * Sockets
 
     * :kconfig:option:`CONFIG_NET_SOCKETS_INET_RAW`

doc/services/pm/device_runtime.rst

@@ -107,9 +107,26 @@ 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, runtime PM operations are offloaded to the system work queue.
+However, device drivers must not perform any blocking operations during suspend, as
+this can stall the system work queue and negatively impact system responsiveness.
+
+To address this, applications can configure runtime PM to use a dedicated work queue
+by enabling :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME_USE_DEDICATED_WQ`.
+
+If blocking behavior is required—for example, when accessing a slow peripheral
+or waiting for a bus transaction—the PM subsystem work queue must be used instead.
+Drivers that require this behavior can explicitly request it by enabling
+:kconfig:option:`CONFIG_PM_DRIVER_NEEDS_DEDICATED_WQ`.
+
+For targets with constrained resources that do not need asynchronous
+operations, this functionality can be disabled altogether by
+de-selecting :kconfig:option:`CONFIOG_PM_DEVICE_RUNTIME_ASYNC`, reducing
+memory usage and system complexity.
+
 
 .. figure:: images/devr-async-ops.svg