Add note to restart the dev server after changing .env file (#6979)

* Add note to restart the dev server after change .env file

* Update copy

Co-authored-by: Ian Sutherland <ian@iansutherland.ca>

Author: MostafaNawara

docusaurus/docs/adding-custom-environment-variables.md

@@ -117,6 +117,8 @@ REACT_APP_NOT_SECRET_CODE=abcdef
 
 > Note: You must create custom environment variables beginning with `REACT_APP_`. Any other variables except `NODE_ENV` will be ignored to avoid [accidentally exposing a private key on the machine that could have the same name](https://github.com/facebook/create-react-app/issues/865#issuecomment-252199527). Changing any environment variables will require you to restart the development server if it is running.
 
+> Note: You need to restart the development server after changing `.env` files.
+
 `.env` files **should be** checked into source control (with the exclusion of `.env*.local`).
 
 ### What other `.env` files can be used?