Add notes to modules_installing/publishing re new PE pages.

Due to the PE 3.3.1 release fixing a PMT bug, the workflow for PE 3.3.1 users has diverged from Puppet 3.6 users. New pages have already been added to PE-specific docs. This adds notes pointing to those docs.
Merge pull request #367 from laurenrother/pe3.3.1

Add notes to modules_installing/publishing re new PE pages.