docs: clearly state that system.internal-url-prefix shouldn't be changed

sentry/config.example.yml

@@ -45,8 +45,16 @@ mail.host: 'smtp'
 # System Settings #
 ###################
 
-# The URL prefix in which Sentry is accessible
+# This is the main URL prefix where Sentry can be accessed.
+# Sentry will use this to create links to its different parts of the user interface.
+# This is most helpful if you are using an external reverse proxy.
 # system.url-prefix: https://example.sentry.com
+
+# Most of the time, this should NOT be changed. It's used for communication
+# between containers. `web` is the container's name, and `9000` is the
+# default port exposed by the Sentry image. If you want to change the
+# publicly exposed domain or port, you should probably change
+# `system.url-prefix` instead.
 system.internal-url-prefix: 'http://web:9000'
 
 # If this file ever becomes compromised, it's important to generate a new key.