Document how to configure the PostgreSQL connection to use TLS/SSL

https://doc.nuxeo.com/nxdoc/postgresql/ could include pointers to the PostgreSQL documentation about configuring the JDBC driver to use SSL (https://jdbc.postgresql.org/documentation/head/ssl-client.html) and the sslcert, sslkey, and sslrootcert connection parameters with sslmode=verify-full.