Hooks/Docs: enqueue_block_assets in block-editor.php missing xref to script-loader.php

## Problem

`do_action( 'enqueue_block_assets' )` in `wp-includes/block-editor.php:341` has no PHPDoc or cross-reference comment. The canonical PHPDoc lives in `wp-includes/script-loader.php:2509-2516`.

## Fix

Add `/** This action is documented in wp-includes/script-loader.php */` above the call.

Related to https://core.trac.wordpress.org/ticket/64224