docs: document webkitdirectory breaking change (#21178)

Manually backport #20934. See that PR for details.

docs/api/breaking-changes.md

@@ -68,6 +68,40 @@ webFrame.setIsolatedWorldInfo(
 
 This property was removed in Chromium 77, and as such is no longer available.
 
+### `webkitdirectory` attribute for `<input type="file"/>`
+
+The `webkitdirectory` property on HTML file inputs allows them to select folders.
+Previous versions of Electron had an incorrect implementation where the `event.target.files`
+of the input returned a `FileList` that returned one `File` corresponding to the selected folder.
+
+As of Electron 7, that `FileList` is now list of all files contained within
+the folder, similarly to Chrome, Firefox, and Edge
+([link to MDN docs](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/webkitdirectory)).
+
+As an illustration, take a folder with this structure:
+```console
+folder
+├── file1
+├── file2
+└── file3
+```
+
+In Electron <=6, this would return a `FileList` with a `File` object for:
+```console
+path/to/folder
+```
+
+In Electron 7, this now returns a `FileList` with a `File` object for:
+```console
+/path/to/folder/file3
+/path/to/folder/file2
+/path/to/folder/file1
+```
+
+Note that `webkitdirectory` no longer exposes the path to the selected folder.
+If you require the path to the selected folder rather than the folder contents,
+see the `dialog.showOpenDialog` API ([link](https://github.com/electron/electron/blob/master/docs/api/dialog.md#dialogshowopendialogbrowserwindow-options)).
+
 ## Planned Breaking API Changes (6.0)
 
 ### `win.setMenu(null)`
@@ -197,7 +231,7 @@ A new API, `protocol.registerSchemesAsPrivileged` has been added and should be u
 ### webFrame Isolated World APIs
 
 ```js
-// Deprecated
+// Removed in Electron 7.0
 webFrame.setIsolatedWorldContentSecurityPolicy(worldId, csp)
 webFrame.setIsolatedWorldHumanReadableName(worldId, name)
 webFrame.setIsolatedWorldSecurityOrigin(worldId, securityOrigin)