updated docs about ansible_hosts/inventory env vars

docsite/rst/intro_installation.rst

@@ -143,7 +143,11 @@ other than /etc/ansible/hosts:
 .. code-block:: bash
 
     $ echo "127.0.0.1" > ~/ansible_hosts
-    $ export ANSIBLE_HOSTS=~/ansible_hosts
+    $ export ANSIBLE_INVENTORY=~/ansible_hosts
+
+.. note::
+
+    ANSIBLE_INVENTORY is available starting at 1.9 and subtitutes the deprecated ANSIBLE_HOSTS
 
 You can read more about the inventory file in later parts of the manual.