doc: document "shutdown" section in autoinstall-reference

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

documentation/autoinstall-reference.md

@@ -473,6 +473,7 @@ The timezone to configure on the system. The special value "geoip" can be used t
 
 **type:** string (enumeration)
 **default:** `security`
+**can be interactive:** no
 
 The type of updates that will be downloaded and installed after the system install.
 Supported values are:
@@ -480,6 +481,18 @@ Supported values are:
  * `security` -> download and install updates from the -security pocket
  * `all` -> also download and install updates from the -updates pocket
 
+### shutdown
+
+**type:** string (enumeration)
+**default:** do nothing
+**can be interactive:** no
+
+Request the system to shutdown or reboot automatically after the installation has finished.
+Supported values are:
+
+ * `reboot`
+ * `shutdown`
+
 <a name="late-commands"></a>
 
 ### late-commands