docs: add warning on client side environment variables (#5243)

Inspired by the create-react-app documentation
https://create-react-app.dev/docs/adding-custom-environment-variables

docs/guide/mode-and-env.md

@@ -43,10 +43,16 @@ An env file simply contains key=value pairs of environment variables:
 
 ```
 FOO=bar
-VUE_APP_SECRET=secret
+VUE_APP_NOT_SECRET_CODE=some_value
 ```
 
-Note that only variables that start with `VUE_APP_` will be statically embedded into the client bundle with `webpack.DefinePlugin`.
+::: warning
+Do not store any secrets (such as private API keys) in your app!
+
+Environment variables are embedded into the build, meaning anyone can view them by inspecting your app's files.
+:::
+
+Note that only `NODE_ENV`, `BASE_URL`, and variables that start with `VUE_APP_` will be statically embedded into the *client bundle* with `webpack.DefinePlugin`. It is to avoid accidentally exposing a private key on the machine that could have the same name.
 
 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+).
 
@@ -87,10 +93,10 @@ In both cases, the app is built as a production app because of the `NODE_ENV`, b
 You can access env variables in your application code:
 
 ``` js
-console.log(process.env.VUE_APP_SECRET)
+console.log(process.env.VUE_APP_NOT_SECRET_CODE)
 ```
 
-During build, `process.env.VUE_APP_SECRET` will be replaced by the corresponding value. In the case of `VUE_APP_SECRET=secret`, it will be replaced by `"secret"`.
+During build, `process.env.VUE_APP_NOT_SECRET_CODE` will be replaced by the corresponding value. In the case of `VUE_APP_NOT_SECRET_CODE=some_value`, it will be replaced by `"some_value"`.
 
 In addition to `VUE_APP_*` variables, there are also two special variables that will always be available in your app code: