Add missing configuration documentation

The admin_log_auth_failures parameter was not documented.

Documentation/Getting-Started/Configuration-Guide.md

@@ -594,6 +594,11 @@ documentation for more details.
 Enable or disable the admin interface. This allows the admin interface to
 be completely disabled to prevent access to it.
 
+#### `admin_log_auth_failures`
+
+Log authentication failures for the admin interface. This parameter expects a
+boolean value and is enabled by default.
+
 #### `sql_mode`
 
 Specifies whether the query classifier parser should initially expect _MariaDB_

maxctrl/lib/alter.js

@@ -34,7 +34,8 @@ const maxscale_params = [
     'auth_connect_timeout',
     'auth_read_timeout',
     'auth_write_timeout',
-    'admin_auth'
+    'admin_auth',
+    'admin_log_auth_failures'
 ]
 
 exports.command = 'alter <command>'