docs: migration guide for #18070 (#18519)

Author: sapphi-red

docs/guide/migration.md

@@ -58,6 +58,8 @@ There are other breaking changes which only affect few users.
 
 - [[#15637] fix!: default `build.cssMinify` to `'esbuild'` for SSR](https://github.com/vitejs/vite/pull/15637)
   - [`build.cssMinify`](/config/build-options#build-cssminify) is now enabled by default even for SSR builds.
+- [[#18070] feat!: proxy bypass with WebSocket](https://github.com/vitejs/vite/pull/18070)
+  - `server.proxy[path].bypass` is now called for WebSocket upgrade requests and in that case, the `res` parameter will be `undefined`.
 - [[#18209] refactor!: bump minimal terser version to 5.16.0](https://github.com/vitejs/vite/pull/18209)
   - Minimal supported terser version for [`build.minify: 'terser'`](/config/build-options#build-minify) was bumped to 5.16.0 from 5.4.0.
 - [[#18231] chore(deps): update dependency @rollup/plugin-commonjs to v28](https://github.com/vitejs/vite/pull/18231)