docs: note env in command line (#7507)

docs/guide/env-and-mode.md

@@ -37,7 +37,7 @@ Vite uses [dotenv](https://github.com/motdotla/dotenv) to load additional enviro
 
 An env file for a specific mode (e.g. `.env.production`) will take higher priority than a generic one (e.g. `.env`).
 
-In addition, environment variables that already exist when Vite is executed have the highest priority and will not be overwritten by `.env` files.
+In addition, environment variables that already exist when Vite is executed have the highest priority and will not be overwritten by `.env` files. For example, when running `VITE_SOME_KEY=123 vite build`.
 
 `.env` files are loaded at the start of Vite. Restart the server after making changes.
 :::