Update Host Maintenance strategy documentation

Add clarifications to the documentation to reflect
the actual strategy usage, including:
 - updating parameter descriptions
 - extending the 'How to Use' section

Closes-Bug: #2111810
Change-Id: Ifd2876056cd8819c50658fb9f213246dc1546d42

doc/source/strategies/host_maintenance.rst

@@ -11,10 +11,6 @@ Synopsis
 
     .. watcher-term:: watcher.decision_engine.strategy.strategies.host_maintenance.HostMaintenance
 
-Requirements
-------------
-
-None.
 
 Metrics
 *******
@@ -56,15 +52,15 @@ Configuration
 
 Strategy parameters are:
 
-==================== ====== ====================================
-parameter            type   default Value description
-==================== ====== ====================================
-``maintenance_node`` String The name of the compute node which
-                            need maintenance. Required.
-``backup_node``      String The name of the compute node which
-                            will backup the maintenance node.
-                            Optional.
-==================== ====== ====================================
+==================== ====== ================================ ==================
+parameter            type   description                      required/optional
+==================== ====== ================================ ==================
+``maintenance_node`` String The name of the compute node     Required
+                            which needs maintenance.
+``backup_node``      String The name of the compute node     Optional
+                            which will backup the
+                            maintenance node.
+==================== ====== ================================ ==================
 
 Efficacy Indicator
 ------------------
@@ -80,13 +76,34 @@ to: https://specs.openstack.org/openstack/watcher-specs/specs/queens/approved/cl
 How to use it ?
 ---------------
 
+Run an audit using Host Maintenance strategy.
+Executing the actions will move the servers from compute01 host
+to a host determined by the Nova scheduler service.
+
+.. code-block:: shell
+
+    $ openstack optimize audit create \
+      -g cluster_maintaining -s host_maintenance \
+      -p maintenance_node=compute01
+
+Run an audit using Host Maintenance strategy with a backup node specified.
+Executing the actions will move the servers from compute01 host
+to compute02 host.
+
 .. code-block:: shell
 
     $ openstack optimize audit create \
       -g cluster_maintaining -s host_maintenance \
       -p maintenance_node=compute01 \
-      -p backup_node=compute02 \
-      --auto-trigger
+      -p backup_node=compute02
+
+Note that after executing this strategy, the *maintenance_node* will be
+marked as disabled, with the reason set to ``watcher_maintaining``.
+To enable the node again:
+
+.. code-block:: shell
+
+   $ openstack compute service set --enable compute01
 
 External Links
 --------------