Docs: Mentioned the fact that UI update should be called manually for…

… UIElements.

src/view/observer/mutationobserver.js

@@ -244,7 +244,7 @@ export default class MutationObserver extends Observer {
 			}
 		}
 
-		// In case only non-relevant mutations were recorded it skips the event & force render (#5600).
+		// In case only non-relevant mutations were recorded it skips the event and force render (#5600).
 		if ( viewMutations.length ) {
 			this.document.fire( 'mutations', viewMutations, viewSelection );
 

src/view/uielement.js

@@ -123,6 +123,9 @@ export default class UIElement extends Element {
 	 *			return domElement;
 	 *		};
 	 *
+	 * If you wish to trigger render of entire editor UI you should call {@link module:core/editor/editorui~EditorUI#update} method
+	 * after rendering your UI element.
+	 *
 	 * @param {Document} domDocument
 	 * @returns {HTMLElement}
 	 */