Add detail into doc

docs/migration-guide.md

@@ -29,9 +29,16 @@ dependencies {
 }
 ```
 
-### LcpDialogAuthentication now supports configuration changes. 
+### LcpDialogAuthentication now supports configuration changes.
 
-See documentation.
+You no longer need to pass an activity, fragment or view as `sender` to `retrievePassphrase`.
+Instead, call `onParentViewAttachedToWindow` every time you have a
+view attached to a window available as anchor and `onParentViewDetachedFromWindow` every time it
+gets detached. You can monitor these events by setting a `View.OnAttachStateChangeListener` on your
+view.
+
+See the [test-app](https://github.com/readium/kotlin-toolkit/blob/01d6c7936accea2d6b953d435e669260676e8c99/test-app/src/main/java/org/readium/r2/testapp/bookshelf/BookshelfFragment.kt#L68)
+for an example.
 
 ### All resources now have the prefix `readium_`.