docs: clarify .env loading precedence / behavior (#18765)

docs/guide/env-and-mode.md

@@ -29,6 +29,8 @@ 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`).
 
+Vite will always load `.env` and `.env.local` in addition to the mode-specific `.env.[mode]` file. Variables declared in mode-specific files will take precedence over those in generic files, but variables defined only in `.env` or `.env.local` will still be available in the environment.
+
 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.