Clarify purpose of ROOT_URL (#3759)

Author: ad-si

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -84,7 +84,9 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 
 - `PROTOCOL`: **http**: \[http, https, fcgi, unix\]
 - `DOMAIN`: **localhost**: Domain name of this server.
-- `ROOT_URL`: **%(PROTOCOL)s://%(DOMAIN)s:%(HTTP\_PORT)s/**: Full public URL of Gitea server.
+- `ROOT_URL`: **%(PROTOCOL)s://%(DOMAIN)s:%(HTTP\_PORT)s/**:
+   Overwrite the automatically generated public URL.
+   This is useful if the internal and the external URL don't match (e.g. in Docker).
 - `HTTP_ADDR`: **0.0.0.0**: HTTP listen address.
    - If `PROTOCOL` is set to `fcgi`, Gitea will listen for FastCGI requests on TCP socket
      defined by `HTTP_ADDR` and `HTTP_PORT` configuration settings.