Update mode-and-env.md (#4257)

* Update mode-and-env.md

Fixed example of keys, which they should start with VUE_APP_

* Update mode-and-env.md

change position of the tip

* Update docs/guide/mode-and-env.md

Co-Authored-By: Natalia Tepluhina <NataliaTepluhina@users.noreply.github.com>
(cherry picked from commit 149b307)

Author: 2 people

docs/guide/mode-and-env.md

@@ -46,6 +46,8 @@ FOO=bar
 VUE_APP_SECRET=secret
 ```
 
+Note that only variables that start with `VUE_APP_` will be statically embedded into the client bundle with `webpack.DefinePlugin`.
+
 For more detailed env parsing rules, please refer to [the documentation of `dotenv`](https://github.com/motdotla/dotenv#rules). We also use [dotenv-expand](https://github.com/motdotla/dotenv-expand) for variable expansion (available in Vue CLI 3.5+).
 
 Loaded variables will become available to all `vue-cli-service` commands, plugins and dependencies.
@@ -82,7 +84,7 @@ In both cases, the app is built as a production app because of the `NODE_ENV`, b
 
 ### Using Env Variables in Client-side Code
 
-Only variables that start with `VUE_APP_` will be statically embedded into the client bundle with `webpack.DefinePlugin`. You can access them in your application code:
+You can access env variables in your application code:
 
 ``` js
 console.log(process.env.VUE_APP_SECRET)