added fact caching hint

docsite/rst/playbooks_variables.rst

@@ -842,7 +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. 
+You need to make sure that the facts of those hosts have been populated before though, for example by running a play against them if the facts have not been cached recently (fact caching was added in Ansible 1.8).
 
 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