Add mapping to a Map<String, ConfigObject> in the documentation

(cherry picked from commit 0c4ab8c)

docs/src/main/asciidoc/config-mappings.adoc

@@ -417,6 +417,39 @@ server.form.landing-page=index.html
 The configuration property needs to specify an additional name to act as the key. In this case the `form()` `Map` will
 contain three elements with the keys `login-page`, `error-page` and `landing-page`.
 
+It also works for groups:
+[source,java]
+----
+@ConfigMapping(prefix = "server")
+public interface Servers {
+    @WithParentName
+    Map<String, Server> allServers();
+}
+
+public interface Server {
+    String host();
+
+    int port();
+
+    String login();
+
+    String error();
+
+    String landing();
+}
+----
+.application.properties
+[source,properties]
+----
+server."my-server".host=localhost
+server."my-server".port=8080
+server."my-server".login=login.html
+server."my-server".error=error.html
+server."my-server".landing=index.html
+----
+In this case the `allServers()` `Map` will
+contain one `Server` element with the key `my-server`.
+
 === Defaults
 
 The `@WithDefault` annotation allows to set a default property into a mapping (and prevent and error if the