doc: nrf: ug_nrf54h20: Explicitly document PM device runtime default

PM device runtime is enabled by default for the H20 if CONFIG_PM is
enabled, this is how unused peripherals are disabled before entering
sleep automatically. Explicitly state that PM device runtime is
enabled by default, and explicitly state that it is PM device runtime
causing the automatic disabling.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>

Author: bjarki-andreasen

doc/nrf/app_dev/device_guides/nrf54h/ug_nrf54h20_pm_optimization.rst

@@ -205,9 +205,12 @@ For applications running on either the application core or the radio core, set t
   * :kconfig:option:`CONFIG_PM` to ``y``
   * :kconfig:option:`CONFIG_POWEROFF` to ``y``
 
+These options automatically enable :kconfig:option:`CONFIG_PM_DEVICE_RUNTIME`, which is required to manage the energy states of individual devices.
+
 Consider also the following recommendations:
 
-  * Disable all unused peripherals before entering sleep (Zephyr's API does this automatically when supported).
+  * Disable all unused peripherals before entering sleep.
+    When supported, Zephyr's API automatically handles this by using the PM device runtime.
   * Build and program an empty image on any unused core to release shared resources.
   * If one or more specific sleep states are not desired, disable them in the devicetree by setting their status to ``disabled``: