chore: Improve FILES_URL Configuration Comments (#19040)

docker/.env.example

 # File preview or download Url prefix.
 # used to display File preview or download Url to the front-end or as Multi-model inputs;
 # Url is signed and has expiration time.
+# Setting FILES_URL is required for file processing plugins.
+#   - For https://example.com, use FILES_URL=https://example.com
+#   - For http://example.com, use FILES_URL=http://example.com
+#   Recommendation: use a dedicated domain (e.g., https://upload.example.com).
+#   Alternatively, use http://<your-ip>:5001 or http://api:5001,
+#   ensuring port 5001 is externally accessible (see docker-compose.yaml).
 FILES_URL=
 
 # ------------------------------