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
Adding more unittests for the v2 role class
Refactoring role spec stuff into a dedicated parsing class

Also reworking tests to cut down on the number of patches required
by sub-classing the DataLoader() class and reworking the base object's
structure a bit to allow its use
Make sure v2 playbook classes validate attributes

Also removing cruft code from earlier iteration on task.py and fixing
a bug where 'shell' was not removed from the task ds after munge()
cleaned things up