Add a note that you may not have a class named foo::init.

When migrating from puppet 2.7 to 3.6, I banged my head on a desk for a bit wondering why Puppet was telling me it could not find a class we had named foo::init inside of foo/manifests/init.pp. The class was there. It worked in 2.7. Turns out init is a reserved word and can't be used for class names (wish the error had been more helpful, instead of 'not found.').