doc: Update document for --pm_notify_channel
Update the document for new parameter --pm_notify_channel.
@@ -326,3 +326,14 @@ Here are descriptions for each of these ``acrn-dm`` command line parameters:
level ranges from 1 (``error``) up to 5 (``debug``).
By default, the log severity level is set to 4 (``info``).
+
+ * - :kbd:`--pm_notify_channel <channel>`
+ - This option is used to define which channel could be used DM to
+ communicate with VM about power management event.
+
+ ACRN supports two channels: ``ioc`` and ``power button``.
+
+ usage::
+ --pm_notify_channel ioc
+
+ Use ioc as power management event motify channel.
