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.
added fact caching hint
Merge pull request #9408 from aklamio/devel

Update playbooks_variables.rst