Add note to docs about using Sass and Flow together (#5823)

* Add default values to `file_ext` note

* Update note to include a link to the relevant docs

* Update copy to indicate the default values

* Add missing space

Author: alaycock

docusaurus/docs/adding-a-sass-stylesheet.md

@@ -34,10 +34,14 @@ This will allow you to do imports like
 
 > **Note:** You must prefix imports from `node_modules` with `~` as displayed above.
 
-> **Note:** If you're using Flow, add the following to your `.flowconfig` so it'll recognize the `.sass` or `.scss` imports.
+> **Note:** If you're using Flow, override the [module.file_ext](https://flow.org/en/docs/config/options/#toc-module-file-ext-string) setting in your `.flowconfig` so it'll recognize `.sass` or `.scss` files. You will also need to include the `module.file_ext` default settings for `.js`, `.jsx`, `.mjs` and `.json` files.
 >
 > ```
 > [options]
+> module.file_ext=.js
+> module.file_ext=.jsx
+> module.file_ext=.mjs
+> module.file_ext=.json
 > module.file_ext=.sass
 > module.file_ext=.scss
 > ```