Update puppetDB docs to include info on LibPQFactory

Prior to this commit, the docs only enabled ssl for postgresql
connections. This commit changes the docs to add a few extra lines in
the database.ini file to use host cert/key and CA cert for verification
with ssl.
Sorry mate, one more thing. As this section speaks of public certificates, I'm not even sure this needs to be changed to point at a specific root cert. I should have noticed this earlier, sorry I've just been distracted.