Document @Reducer(state: .equatable) in the migration guide (#2796)

Sources/ComposableArchitecture/Documentation.docc/Articles/MigrationGuides/MigratingTo1.8.md

@@ -90,9 +90,19 @@ enum Destination {
 ```
 
 24 lines of code has become 6. The `@Reducer` macro can now be applied to an _enum_ where each
-case holds onto the reducer that governs the logic and behavior for that case. Further, when
-using the ``Reducer/ifLet(_:action:)`` operator with this style of `Destination` enum reducer
-you can completely leave off the trailing closure as it can be automatically inferred:
+case holds onto the reducer that governs the logic and behavior for that case.
+
+> Note: If the parent feature has equatable state, you must extend the the generated `State` of the
+> enum reducer to be `Equatable` as well. Due to a bug in Swift 5.9 that prevents this from being
+> done in the same file with an explicit extension, we provide the following configuration options,
+> ``Reducer(state:action:)``, instead, which can be told which synthesized conformances to apply:
+>
+> ```swift
+> @Reducer(state: .equatable)
+> ```
+
+Further, when using the ``Reducer/ifLet(_:action:)`` operator with this style of `Destination` enum
+reducer you can completely leave off the trailing closure as it can be automatically inferred:
 
 ```diff
  Reduce { state, action in