docs: addd "collapsed" parameter to "registerEditorComponent" for issue #5779 (#5858)

website/content/docs/custom-widgets.md

@@ -111,6 +111,8 @@ CMS.registerEditorComponent(definition)
 
 * **definition:** The component definition; must specify: id, label, fields, patterns, fromBlock, toBlock, toPreview
 
+> Additional properties are optional and will be passed to the underlying widget control (object widget by default). For example, adding a `collapsed: true` property will collapse the widget by default.
+
 **Example:**
 
 ```html
@@ -559,4 +561,4 @@ npm run build
 
 ```javascript
 npm publish
-```
+```