Add note on 'UVICORN_' prefixed settings and '--env-file' (#1372)

* Add note on 'UVICORN_' prefixed settings and '--env-file'

* Update settings.md

* Update settings.md

docs/settings.md

@@ -13,6 +13,7 @@ For example, in case you want to run the app on port `5000`, just set the enviro
 !!! note
     CLI options and the arguments for `uvicorn.run()` take precedence over environment variables.
 
+    Also note that `UVICORN_*` prefixed settings cannot be used from within an environment configuration file. Using an environment configuration file with the `--env-file` flag is intended for configuring the ASGI application that uvicorn runs, rather than configuring uvicorn itself.
 
 ## Application