Add migration guide for text field focus node change (#6407)

* Add migration guide for text field focus node change

* Update editable-text-focus-attachment.md

* Update editable-text-focus-attachment.md

* Update landed version

* Apply suggestions from code review

Co-authored-by: Shams Zakhour (ignore Sfshaza) <44418985+sfshaza2@users.noreply.github.com>

* Fix master api link

Co-authored-by: Shams Zakhour (ignore Sfshaza) <44418985+sfshaza2@users.noreply.github.com>

Author: LongCatIsLooong

src/docs/release/breaking-changes/editable-text-focus-attachment.md

@@ -0,0 +1,107 @@
+---
+title: TextField FocusNode attach location change
+description: EditableText.focusNode is no longer attached to EditableTextState's BuildContext
+---
+
+## Summary
+
+`EditableText.focusNode` is now attached to a dedicated `Focus` widget below
+`EditableText`.
+
+## Context
+
+A text input field widget (`TextField`, for example) typically owns a `FocusNode`.
+When that `FocusNode` is the primary focus of the app, events (such as key presses) 
+are sent to the `BuildContext` to which the `FocusNode` is attached. 
+
+The `FocusNode` also plays a roll in shortcut handling: The `Shortcuts` widget 
+translates key sequences into an `Intent`, and tries to find the first suitable 
+handler for that `Intent` starting from the `BuildContext` to which the `FocusNode` 
+is attached, to the root of the widget tree. This means an `Actions` widget (that provides 
+handlers for different `Intent`s) won't be able to handle any shortcut `Intent`s 
+when the `BuildContext` that has the primary focus is above it in the tree.
+
+Previously for `EditableText`, the `FocusNode` was attached to the `BuildContext`
+of `EditableTextState`. Any `Actions` widgets defined in `EditableTextState` (which 
+will be inflated below the `BuildContext` of the `EditableTextState`) couldn't 
+handle shortcuts even when that `EditableText` was focused, for the reason stated 
+above.
+
+## Description of change
+
+`EditableTextState` now creates a dedicated `Focus` widget to host `EditableText.focusNode`.
+This allows `EditableTextState`s to define handlers for shortcut `Intent`s. For 
+instance, `EditableText` now has a handler that handles the "deleteCharacter" intent
+when the <kbd>DEL<kbd> key is pressed.
+
+This change does not involve any public API changes but breaks codebases relying on 
+that particular implementation detail to tell if a `FocusNode` is associated with a
+text input field.
+
+This change does not break any builds but can introduce runtime issues, or
+cause existing tests to fail.
+
+## Migration guide
+
+The `EditableText` widget takes a `FocusNode` as a paramter, which was
+previously attached to its `EditableText`'s `BuildContext`. If you are relying
+on runtime typecheck to find out if a `FocusNode` is attached to a text input
+field or a selectable text field like so:
+
+- `focusNode.context.widget is EditableText`
+- `(focusNode.context as StatefulElement).state as EditableTextState`
+
+Then please read on and consider following the migration steps to avoid breakages.
+
+If you're not sure whether a codebase needs migration, search for `is EditableText`,
+`as EditableText`, `is EditableTextState`, and `as EditableTextState` and verify if
+any of the search results are doing a typecheck or typecast on a `FocusNode.context`.
+If so, then migration is needed.
+
+To avoid performing a typecheck, or downcasting the `BuildContext` associated with the
+`FocusNode` of interest, and depending on the actual capabilities the codebase is
+trying to invoke from the given `FocusNode`, fire an `Intent` from that
+`BuildContext`. For instance, if you wish to update the text of the currently focused
+`TextField` to a specific value, see the following example:
+
+Code before migration:
+
+<!-- skip -->
+```dart
+final Widget? focusedWidget = primaryFocus?.context?.widget;
+if (focusedWidget is EditableText) {
+  widget.controller.text = 'Updated Text';
+}
+```
+
+Code after migration:
+
+<!-- skip -->
+```dart
+final BuildContext? focusedContext = primaryFocus?.context;
+if (focusedContext != null) {
+  Actions.maybeInvoke(focusedContext, ReplaceTextIntent('UpdatedText'));
+}
+```
+
+For a comprehensive list of `Intent`s supported by the `EditableText` widget,
+refer to the documentation of the `EditableText` widget.
+
+## Timeline
+
+Landed in version: 2.6.0-12.0.pre<br>
+In stable release: not yet
+
+## References
+
+API documentation:
+
+* [`EditableText`][]
+
+Relevant PRs:
+
+* [Move text editing Actions to EditableTextState][]
+
+[`EditableText`]: {{site.master-api}}/flutter/widgets/EditableText-class.html
+[Move text editing Actions to EditableTextState]: {{site.github}}/flutter/flutter/pull/90684
+