docs: document notification-mode and merge old notification section

This new section describes how the notification-mode parameter works.
The section also contains also parts of the old notification section
from the maintenance chapter, reusing the description of the
`notify` and `notify-user` parameters.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Gabriel Goller <g.goller@proxmox.com>

docs/maintenance.rst

@@ -277,26 +277,10 @@ the **Actions** column in the table.
 Notifications
 -------------
 
-Proxmox Backup Server can send you notification emails about automatically
+Proxmox Backup Server can send you notifications about automatically
 scheduled verification, garbage-collection and synchronization tasks results.
 
-By default, notifications are sent to the email address configured for the
-`root@pam` user. You can instead set this user for each datastore.
-
-.. image:: images/screenshots/pbs-gui-datastore-options.png
-  :target: _images/pbs-gui-datastore-options.png
-  :align: right
-  :alt: Datastore Options
-
-You can also change the level of notification received per task type, the
-following options are available:
-
-* Always: send a notification for any scheduled task, independent of the
-  outcome
-
-* Errors: send a notification for any scheduled task that results in an error
-
-* Never: do not send any notification at all
+Refer to the :ref:`notifications` chapter for more details.
 
 .. _maintenance_mode:
 

docs/notifications.rst

@@ -1,3 +1,5 @@
+.. _notifications:
+
 Notifications
 =============
 
@@ -33,6 +35,10 @@ the latter contains sensitive configuration options such as
 passwords or authentication tokens for notification targets and
 can only be read by ``root``.
 
+Datastores and tape backup jobs have a configurable :ref:`notification_mode`,
+which allows you to choose between the notification system and a legacy mode
+for sending notification emails.
+
 
 Notification Targets
 --------------------
@@ -211,3 +217,50 @@ Permissions
 In order to modify/view the configuration for notification targets,
 the ``Sys.Modify/Sys.Audit`` permissions are required for the
 ``/system/notifications`` ACL node.
+
+.. _notification_mode:
+
+Notification Mode
+-----------------
+Datastores and tape backup/restore job configuration have a ``notification-mode``
+option which can have one of two values:
+
+* ``legacy-sendmail``: Send notification emails via the system's ``sendmail`` command.
+  The notification system will be bypassed and any configured targets/matchers will be ignored.
+  This mode is equivalent to the notification behavior for version before
+  Proxmox Backup Server 3.2.
+
+* ``notification-system``: Use the new, flexible notification system.
+
+If the ``notification-mode`` option is not set, Proxmox Backup Server will default
+to ``legacy-sendmail``.
+
+Starting with Proxmox Backup Server 3.2, a datastore created in the UI will
+automatically opt in to the new notification system. If the datastore is created
+via the API or the ``proxmox-backup-manager`` CLI, the ``notification-mode``
+option has to be set explicitly to ``notification-system`` if the
+notification system shall be used.
+
+The ``legacy-sendmail`` mode might be removed in a later release of
+Proxmox Backup Server.
+
+Settings for ``legacy-sendmail`` notification mode
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If ``notification-mode`` is set to ``legacy-sendmail``,  Proxmox Backup Server
+will send notification emails via the system's ``sendmail`` command to the email
+address configured for the user set in the ``notify-user`` option
+(falling back to ``root@pam`` if not set).
+
+For datastores, you can also change the level of notifications received per task
+type via the ``notify`` option.
+
+* Always: send a notification for any scheduled task, independent of the
+  outcome
+
+* Errors: send a notification for any scheduled task that results in an error
+
+* Never: do not send any notification at all
+
+The ``notify-user`` and ``notify`` options are ignored if ``notification-mode``
+is set to ``notification-system``.

docs/storage.rst

@@ -314,7 +314,7 @@ Options
 
 There are a few per-datastore options:
 
-* :ref:`Notifications <maintenance_notification>`
+* :ref:`Notification mode and legacy notification settings <notification_mode>`
 * :ref:`Maintenance Mode <maintenance_mode>`
 * Verification of incoming backups
 
@@ -419,7 +419,7 @@ remote-source to avoid that an attacker that took over the source can cause
 deletions of backups on the target hosts.
 If the source-host became victim of a ransomware attack, there is a good chance
 that sync jobs will fail, triggering an :ref:`error notification
-<maintenance_notification>`.
+<Notification Events>`.
 
 It is also possible to create :ref:`tape backups <tape_backup>` as a second
 storage medium. This way, you get an additional copy of your data on a