Update 1.25 migration guide for enum alerts (#3904)

Alerts and confirmation dialogs held in `@Reducer enum`s will require a
bit of extra ceremony to be 2.0-compatible, but 2.0 will come with more
flexible tooling around handling prompts.

Author: stephencelis

Examples/SyncUps/SyncUps/SyncUpDetail.swift

@@ -8,6 +8,12 @@ struct SyncUpDetail {
     case alert(AlertState<Alert>)
     case edit(SyncUpForm)
 
+    @CasePathable
+    enum Action {
+      case alert(Alert)
+      case edit(SyncUpForm.Action)
+    }
+
     @CasePathable
     enum Alert {
       case confirmDeletion

Package.resolved

@@ -39,9 +39,6 @@ directly to a specific case of a destination enum, you now scope to the entire d
 chain into the individual case:
 
 ```diff
--.alert($store.scope(state: \.destination?.alert, action: \.destination.alert))
-+.alert($store.scope(state: \.$destination, action: \.destination).alert)
-
 -.sheet(item: $store.scope(state: \.destination?.edit, action: \.destination.edit)) {
 +.sheet(item: $store.scope(state: \.$destination, action: \.destination).edit) {
 ```
@@ -62,6 +59,28 @@ bindings to `Bool` bindings:
 +) {
 ```
 
+Another important difference is that holding non-feature state in a destination enum with an
+associated action (such as the action of an `AlertState<Action>`) requires an explicit `Action` enum
+definition:
+
+```diff
+ @Reducer enum Destination {
+   case alert(AlertState<Alert>)
+   case settings(Settings)
+
++  @CasePathable enum Action {
++    case alert(Alert)
++    case settings(Settings.Action)
++  }
+
+   enum Alert {
+     case .deleteTapped
+   }
+ }
+```
+
+ComposableArchitecture 2.0 will have newer tools for handling prompts.
+
 ### Streamlined `onChange` operator
 
 A new overload of the `onChange` operator has been added that directly returns an effect instead