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.
It looks like according to the psql docs, the way to get client authentication is to just place the appropriate certs in the ~/.postgresql dir for the client. Haven't tried it yet, but it seems like most of the configuration is on the server side. Will be testing with PE Console later.