Update the documentation "Running Confluence behind NGINX with SSL"

From a recent ticket provided in the comments section, we have found out that not having protocol inside the server.xml causes synchrony to fail.

Therefore, it's great if we include a reminder in the documentation that having the protocol is important for the success of Collaborative Editing.

https://confluence.atlassian.com/doc/running-confluence-behind-nginx-with-ssl-858772080.html