Update os_server docs when removing an instance (#41963)

When removing an instance via its ID, it is not clearly explained that one should use the `name` parameter.

Also a simple example is provided.

+label: docsite_pr

lib/ansible/modules/cloud/openstack/os_server.py

@@ -27,7 +27,8 @@ description:
 options:
    name:
      description:
-        - Name that has to be given to the instance
+        - Name that has to be given to the instance. It is also possible to
+          specify the ID of the instance instead of its name if I(state) is I(absent).
      required: true
    image:
      description:
@@ -381,6 +382,15 @@ EXAMPLES = '''
           ifdown eth0 && ifup eth0
           {% endraw %}
 
+# Deletes an instance via its ID
+- name: remove an instance
+  hosts: localhost
+  tasks:
+    - name: remove an instance
+      os_server:
+        name: abcdef01-2345-6789-0abc-def0123456789
+        state: absent
+
 '''
 
 from ansible.module_utils.basic import AnsibleModule