Document the new variables for ngingx SSL config

The new variables created to the nginx reverse proxy are properly added
to the documentation.

docs/configuring-playbook-nginx.md

@@ -24,6 +24,29 @@ matrix_nginx_proxy_proxy_matrix_nginx_status_allowed_addresses:
 - 1.1.1.1
 ```
 
+## Adjusting SSL in your server
+
+You can adjust how the SSL is served by the nginx server by setting the `matrix_nginx_proxy_ssl_config`. This is based on the Mozilla Server Side TLS
+Recommended configurations. It changes the TLS Protocol, the SSL Cipher Suites and the `ssl_prefer_server_ciphers` variable of nginx.
+The posible values are:
+
+- "Modern" - For Modern clients that support TLS 1.3, with no need for backwards compatibility
+- "Intermediate" - Recommended configuration for a general-purpose server
+- "Old" - Services accessed by very old clients or libraries, such as Internet Explorer 8 (Windows XP), Java 6, or OpenSSL 0.9.8
+- "Custom" - For defining your own protocols an ciphers
+
+The default is set to `"Intermediate"`.
+
+**Be really carefull when setting it to "Modern"**. This could break the comunication with other matrix servers, limiting your feration posibilities and the
+[Federarion tester](https://federationtester.matrix.org/) won't work.
+
+If you set `matrix_nginx_proxy_ssl_config` to `"Custom"`, you will get three variables that you will be able to set:
+
+- `matrix_nginx_proxy_ssl_protocols`: for specifying the supported TLS protocols.
+- `matrix_nginx_proxy_ssl_prefer_server_ciphers`: for specifying if the server or the client choice when negociating the chipher. It can set to "on" or "off".
+- `matrix_nginx_proxy_ssl_ciphers`: for specifying the SSL Cipher suites used by nginx.
+
+For more information about this variables, check the `roles/matrix-nginx-proxy/defaults/main.yml` file.
 
 ## Synapse + OpenID Connect for Single-Sign-On