Update MediaPlaceholder README.md

This change updates the readme to properly document the `value` property.

See issue here: WordPress#17967

packages/block-editor/src/components/media-placeholder/README.md

@@ -155,9 +155,9 @@ The argument of the callback is an object containing the following properties:
 
 ### value
 
-Media ID (or media IDs if multiple is true) to be selected by default when opening the media library.
+An object or an array of objects that contain media ID (`id` property) to be selected by default when opening the media library.
 
-- Type: `Number|Array`
+- Type: `Object|Array`
 - Required: No
 - Platform: Web