Update playbooks_variables.rst

There is some kind of duplicated content with http://docs.ansible.com/faq.html#how-do-i-loop-over-a-list-of-hosts-in-a-group-inside-of-a-template and this gotcha isn't listed here.

docsite/rst/playbooks_variables.rst

@@ -842,6 +842,7 @@ A frequently used idiom is walking a group to find all IP addresses in that grou
    {% endfor %}
 
 An example of this could include pointing a frontend proxy server to all of the app servers, setting up the correct firewall rules between servers, etc.
+You need to make sure that the facts of those hosts have been populated before though, for example by running a play against them. 
 
 Additionally, *inventory_hostname* is the name of the hostname as configured in Ansible's inventory host file.  This can
 be useful for when you don't want to rely on the discovered hostname `ansible_hostname` or for other mysterious