Clarify how settings are applied once in keystore (#16902)

docs/setup/secure-settings.asciidoc

@@ -44,6 +44,17 @@ can be added using the `add` command:
 bin/kibana-keystore add the.setting.name.to.set
 ----------------------------------------------------------------
 
+Once added to the keystore, these setting will be automatically applied
+to this instance of Kibana when started. For example if you do
+
+[source,sh]
+----------------------------------------------------------------
+bin/kibana-keystore add elasticsearch.username
+----------------------------------------------------------------
+
+you will be prompted to provide the value for elasticsearch.username.
+(Your input will show as asterisks.)
+
 The tool will prompt for the value of the setting. To pass the value
 through stdin, use the `--stdin` flag: