Clarify webserver group membership requirement

Discussed in spantaleev#412 (Github Issue).

docs/configuring-playbook-own-webserver.md

@@ -22,7 +22,7 @@ For an alternative, make sure to check Method #2 as well.
 
 No matter which external webserver you decide to go with, you'll need to:
 
-1) Make sure your web server user (something like `http`, `apache`, `www-data`, `nginx`) is part of the `matrix` group. You should run something like this: `usermod -a -G matrix nginx`
+1) Make sure your web server user (something like `http`, `apache`, `www-data`, `nginx`) is part of the `matrix` group. You should run something like this: `usermod -a -G matrix nginx`. This allows your webserver user to access files owned by the `matrix` group. When using an external nginx webserver, this allows it to read configuration files from `/matrix/nginx-proxy/conf.d`. When using another server, it would make other files, such as `/matrix/static-files/.well-known`, accessible to it.
 
 2) Edit your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`) to disable the integrated nginx server: