(MODULES-1221) Add file_line autorequire documentation
(MODULES-1221) Add file_line autorequire documentation

This commit adds additional documentation to the file_line resource
explaining how it will autorequire file resources when present.

yadavnikhil/puppetlabs-stdlib,jbondpdx/puppetlabs-stdlib,puppetlabs/puppetlabs-stdlib,DavidS/puppetlabs-stdlib,fuel-infra/puppetlabs-stdlib,HelenCampbell/puppetlabs-stdlib