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 the ability to keep aliased and deprecated modules prefixed with
'_', they will be loaded after non prefixed modules are checked they can
be full modules or symlinks to existing ones (alias)

also updated ansible doc to ignore these, will eventually add selective
display
added fact caching hint
vars_files haven't been discussed yet

I think this is a chapter ordering thing... vars_files have not actually be explained yet (aside from a mention of them in the note right above, which also might need reconsidering?) I think they do get introduced in the next section 'Variables'.
module formatter skips modules with leading underscore to avoid
documenting them. Soon will be patched to recognize them as either
deprecated or an alias
added info about new deprecated/alias plugin loading
Add a changelog entry for how the git module has changed.
Pull in git submodule tracking
Add branch info for our submodules
Test the git changes for the git module's recursive flag
Remove test case that wasn't reported as something git should do
Fix up all python3 issues that do not have to do with text/bytes
Add copyright and py3 compat header
Add the python3 compat header
Add python3 compat header
As per yum module documents it support 'present, absent and latest'
Current intro doc is pointing older way of using yum module.
Merge pull request #9444 from praveenkumar/devel

As per yum module documents it state only support 'present, absent and latest'
Add the default Homebrew path for OpenSSL certs on OS X
fixed typo
Merge pull request #9408 from aklamio/devel

Update playbooks_variables.rst