docs: add migration hint for alias resolving (#1348)

docs/guide/migration.md

@@ -36,6 +36,17 @@
 
 - All Vue specific options are removed; Pass options to the Vue plugin instead.
 
+## Alias Behavior Change
+
+[`alias`](/config/#alias) is now being passed to `@rollup/plugin-alias` and no longer require start/ending slashes. The behavior is now a direct replacement, so 1.0-style directory aliases now require an ending slash:
+
+```diff
+- alias: { '/@foo/': path.resolve(__dirname, 'some-special-dir') }
++ alias: { '/@foo/': path.resolve(__dirname, 'some-special-dir') + '/' }
+```
+
+Alternatively, you can use the `[{ find: RegExp, replacement: string }]` option format for more precise control.
+
 ## Vue Support
 
 Vite 2.0 core is now framework agnostic. Vue support is now provided via [`@vitejs/plugin-vue`](https://github.com/vitejs/vite/tree/main/packages/plugin-vue). Simply install it and add it in the Vite config: