Doc: Add NOTE about hostname modules does not modify /etc/hosts

lib/ansible/modules/system/hostname.py

@@ -28,9 +28,10 @@ version_added: "1.4"
 short_description: Manage hostname
 requirements: [ hostname ]
 description:
-    - Set system's hostname
+    - Set system's hostname.
     - Currently implemented on Debian, Ubuntu, Fedora, RedHat, openSUSE, Linaro, ScientificLinux, Arch, CentOS, AMI.
-    - Any distribution that uses systemd as their init system
+    - Any distribution that uses systemd as their init system.
+    - NOTE: This module does NOT modify /etc/hosts. You need to modify it yourself using other modules like template or replace.
 options:
     name:
         required: true