improve documentation for nextcloud security

The default configuration does not pass security checks.
A change to ssl.conf is required for secure operation.
This behaviour can be very confusing to new users.
Documenting this should help make it easier for
new nextcloud users to have a secure experience.

nextcloud.subfolder.conf.sample

@@ -34,6 +34,7 @@ location ^~ /nextcloud/ {
     proxy_pass $upstream_proto://$upstream_app:$upstream_port;
 
     rewrite /nextcloud(.*) $1 break;
+    # Uncomment X-Frame-Options directive in ssl.conf to pass security checks.
     proxy_hide_header X-Frame-Options;
     proxy_max_temp_file_size 2048m;
     proxy_set_header Range $http_range;