update ssl documentation

Author: fanny-jiang

manifests/integrations/postgres.pp

@@ -14,7 +14,19 @@
 #   $username
 #       The username for the datadog user
 #   $ssl
-#       Boolean to enable SSL
+#       This option determines whether or not and with what priority a secure SSL TCP/IP connection
+#         is negotiated with the server. There are six modes:
+#         - `disable`: Only tries a non-SSL connection.
+#         - `allow`: First tries a non-SSL connection; if if fails, tries an SSL connection.
+#         - `prefer`: First tries an SSL connection; if it fails, tries a non-SSL connection.
+#         - `require`: Only tries an SSL connection. If a root CA file is present, verifies the certificate in
+#                      the same way as if verify-ca was specified.
+#         - `verify-ca`: Only tries an SSL connection, and verifies that the server certificate is issued by a
+#                        trusted certificate authority (CA).
+#         - `verify-full`: Only tries an SSL connection and verifies that the server certificate is issued by a
+#                          trusted CA and that the requested server host name matches the one in the certificate.
+#
+#         For a detailed description of how these options work see https://www.postgresql.org/docs/current/libpq-ssl.html
 #   $use_psycopg2
 #       Boolean to flag connecting to postgres with psycopg2 instead of pg8000.
 #       Warning, psycopg2 doesn't support ssl mode.