docs: mention caveat about missing plugins when in legacy config (#3857)

* docs: mention caveat about missing plugins
when in legacy config
close #3845

* Update browser-compatibility.md

(cherry picked from commit 09ce29f)

docs/guide/browser-compatibility.md

@@ -89,6 +89,12 @@ Vue CLI uses two environment variables to communicate this:
 **Important:** These variables are only accessible when/after `chainWebpack()` and `configureWebpack()` functions are evaluated, (so not directly in the `vue.config.js` module's root scope). That means it's also available in the postcss config file.
 :::
 
+::: warning Caveat: Adjusting webpack plugins
+Some Plugins, i.e. `html-webpack-plugin`, `preload-plugin` etc. are only included in the config for modern mode. Trying to tap into their options in the legacy config can throw an error as the plugins don't exist.
+
+Use the above tip about *Detecting the Current Mode* to manipulate plugins in the right mode only, and/or check if the plugin actually exists in the current mode's config before trying to tap into their options.
+:::
+
 [autoprefixer]: https://github.com/postcss/autoprefixer
 [babel-preset-env]: https://new.babeljs.io/docs/en/next/babel-preset-env.html
 [babel-preset-app]: https://github.com/vuejs/vue-cli/tree/dev/packages/%40vue/babel-preset-app