Update ironic-inspector usage document

1. Add inspect command at "Node States" section
2. Add rule import command at "Introspection Rules" section

Change-Id: Ie8eb650f1258933bee0d74eddd77a74a73078f96

doc/source/user/usage.rst

@@ -26,6 +26,10 @@ Node States
 
     openstack baremetal node manage <node>
 
+* The introspection can be triggered by using the following command::
+
+    openstack baremetal node inspect <node>
+
 * After successful introspection and before deploying nodes should be made
   available to Nova, by moving them to ``AVAILABLE`` state::
 
@@ -48,6 +52,10 @@ them automatically after running all processing hooks.
 A rule consists of conditions to check, and actions to run. If conditions
 evaluate to true on the introspection data, then actions are run on a node.
 
+Please refer to the command below to import introspection rule::
+
+    openstack baremetal introspection rule import <json file>
+
 Available conditions and actions are defined by plugins, and can be extended,
 see :ref:`contributing_link` for details. See :ref:`api <http_api>` for
 specific calls to define introspection rules.