[postgres] improve docs (#28553)

Signed-off-by: Raul Gutierrez Segales <rsegales@nvidia.com>

Mirrored from https://github.com/envoyproxy/envoy @ 59e0d299ee4715bde3d84171bfaeac5f478dd4b1

contrib/envoy/extensions/filters/network/postgres_proxy/v3alpha/postgres_proxy.proto

@@ -55,6 +55,8 @@ message PostgresProxy {
   // Postgres filter is able to read Postgres payload in clear-text. It happens when
   // a client established a clear-text connection to Envoy or when a client established
   // SSL connection to Envoy and Postgres filter is configured to terminate SSL.
-  // Defaults to SSL_DISABLE.
+  // In order for upstream encryption to work, the corresponding cluster must be configured to use
+  // :ref:`starttls transport socket <envoy_v3_api_msg_extensions.transport_sockets.starttls.v3.UpstreamStartTlsConfig>`.
+  // Defaults to ``SSL_DISABLE``.
   SSLMode upstream_ssl = 4;
 }