docs: mention Custom type

docs/guide/migration.md

@@ -29,6 +29,19 @@ test('validation works', () => {
 }, 1000) // Ok ✅
 ```
 
+### `Custom` Type is Deprecated <Badge type="warning">experimental API</Badge> {#custom-type-is-deprecated}
+
+The `Custom` type is now equal to the `Test` type. Note that Vitest updated the public types in 2.1 and changed exported names to `RunnerCustomCase` and `RunnerTestCase`:
+
+```ts
+import {
+  RunnerCustomCase, // [!code --]
+  RunnerTestCase, // [!code ++]
+} from 'vitest'
+```
+
+If you are using `getCurrentSuite().custom()`, the `type` of the returned task is now is equal to `'test'`. The `Custom` type will be removed in Vitest 4.
+
 ## Migrating to Vitest 2.0
 
 ### Default Pool is `forks`