docs: add a warning about aliasing (vitest-dev#5618)

docs/config/index.md

@@ -318,6 +318,14 @@ To provide object via CLI command, use the following syntax: `--outputFile.json=
 
 Define custom aliases when running inside tests. They will be merged with aliases from `resolve.alias`.
 
+::: warning
+Vitest uses Vite SSR primitives to run tests which has [certain pitfalls](https://vitejs.dev/guide/ssr.html#ssr-externals).
+
+1. Aliases affect only modules imported directly with an `import` keyword by an [inlined](#server-deps-inline) module (all source code is inlined by default).
+2. Vitest does not support aliasing `require` calls.
+3. If you are aliasing an external dependency (e.g., `react` -> `preact`), you may want to alias the actual `node_modules` packages instead to make it work for externalized dependencies. Both [Yarn](https://classic.yarnpkg.com/en/docs/cli/add/#toc-yarn-add-alias) and [pnpm](https://pnpm.io/aliases/) support aliasing via the `npm:` prefix.
+:::
+
 ### globals
 
 - **Type:** `boolean`