doc/userguide: add info about --set and lists

doc/userguide/partials/options.rst

@@ -192,11 +192,26 @@
 .. option:: --set <key>=<value>
 
    Set a configuration value. Useful for overriding basic
-   configuration parameters in the configuration. For example, to
-   change the default log directory::
+   configuration parameters. For example, to change the default log
+   directory::
 
      --set default-log-dir=/var/tmp
 
+   This option cannot be used to add new entries to a list in the
+   configuration file, such as a new output. It can only be used to
+   modify a value in a list that already exists.
+
+   For example, to disable the ``eve-log`` in the default
+   configuration file::
+
+     --set outputs.1.eve-log.enabled=no
+
+   Also note that the index values may change as the ``suricata.yaml``
+   is updated.
+
+   See the output of ``--dump-config`` for existing values that could
+   be modified with their index.
+
 .. option:: --engine-analysis
 
    Print reports on analysis of different sections in the engine and