[doc] Add example to cross-plugin-concepts (#12672)

Adds an example showing users how to use the environment variable to define multiple whitespace-delimited URIs

docs/static/cross-plugin-concepts.asciidoc

@@ -10,8 +10,17 @@ List-type URI parameters will automatically expand strings that contain multiple
 whitespace-delimited URIs into separate entries. This behaviour enables the expansion
 of an arbitrary list of URIs from a single Environment- or Keystore-variable.
 
-Examples of plugins and options that support this functionality:
+These plugins and options support this functionality:
 
 * <<plugins-inputs-elasticsearch-hosts,Elasticsearch input plugin - `hosts`>>
 * <<plugins-outputs-elasticsearch-hosts,Elasticsearch output plugin - `hosts`>>
 * <<plugins-filters-elasticsearch-hosts,Elasticsearch filter plugin - `hosts`>>
+
+You can use this functionality to define an environment variable with
+multiple whitespace-delimited URIs and use it for the options above.
+
+**Example**
+
+```
+ES_HOSTS="es1.example.com es2.example.com:9201 es3.example.com:9201"
+```