Add docs for the batch mode of plugin installation (#21700)

* Add docs for the batch mode of plugin installation

Closes https://github.com/elastic/elasticsearch/issues/21490

* Updated to clarify that all permissions are granted automatically in batch mode

docs/plugins/plugin-script.asciidoc

@@ -126,6 +126,24 @@ return the following exit codes:
 `74`:: IO error
 `70`:: any other error
 
+[float]
+=== Batch mode
+
+Certain plugins require more privileges than those provided by default in core
+Elasticsearch. These plugins will list the required privileges and ask the
+user for confirmation before continuing with installation.
+
+When running the plugin install script from another program (e.g. install
+automation scripts), the plugin script should detect that it is not being
+called from the console and skip the confirmation response, automatically
+granting all requested permissions.  If console detection fails, then batch
+mode can be forced by specifying `-b` or `--batch` as follows:
+
+[source,shell]
+-----------------------------------
+sudo bin/elasticsearch-plugin install --batch [pluginname]
+-----------------------------------
+
 [float]
 === Custom config directory