Couple of points:

* The default location you have specified is a tricky one. Many different distros move this file around, and if you are running a specific version, it might even have the version in the path. You can see the amount of variations for at least Debian/Ubuntu/Redhat here in the module params.pp: https://github.com/kbarber/puppetlabs-postgresql/blob/a8f86f9df107dec32183b9284bdb8fd03044e27f/manifests/params.pp ... you could provide the many places where this can be found based on OS, or you could punt the problem and say 'look in your postgresql configuration dir' and let the user find it. I can't think of a clever way to do this right now.
* I think the description on how to change pg_hba.conf is too many words, not enough examples. I wouldn't advise users to 'change all values of METHOD at the bottom to md5' because it presumes a particular layout. Instead I would say something like 'Ensure there is a rule line allowing MD5 authentication for at least localhost' and provide a full example `pg_hba.conf` (because ordering is important, and an earlier reject line might stop a later auth line) making sure we are clear to say it is an example.