document `provider` parameter for `directories`
document `provider` parameter for `directories`

In directories we can set provider to either directory (default), files,
or location to create different kinds of blocks. At the time this
feature was added, we neglected to document it. This commit fixes #372

traylenator/puppetlabs-apache,buzzdeee/puppetlabs-apache,puppetlabs/puppetlabs-apache,bmjen/puppetlabs-apache,gerhardsam/puppetlabs-apache,gerhardsam/puppetlabs-apache,svanhart/puppetlabs-apache,bmjen/puppetlabs-apache,eputnam/puppetlabs-apache,dzmitryv/puppetlabs-apache,dzmitryv/puppetlabs-apache,traylenator/puppetlabs-apache,puppetlabs/puppetlabs-apache,digitalmediacenter/puppetlabs-apache,FlatKey/puppetlabs-apache,FlatKey/puppetlabs-apache,yakatz/puppetlabs-apache,digitalmediacenter/puppetlabs-apache,samuelb/puppetlabs-apache,guegue/puppetlabs-apache,svanhart/puppetlabs-apache,eputnam/puppetlabs-apache,yakatz/puppetlabs-apache,buzzdeee/puppetlabs-apache,guegue/puppetlabs-apache,samuelb/puppetlabs-apache