Add dockblock to highligh removed functionality.

Author: Andi Rückauer

docs/book/classmap-generator.md

@@ -1,5 +1,8 @@
 # The Class Map Generator utility: bin/classmap\_generator.php
 
-The script `bin/classmap_generator.php` has been removed and is no longer available. Instead, we recommend using [Composer for autoloading](https://getcomposer.org/doc/04-schema.md#autoload).
+> ### Removed functionality
+> 
+> The script `bin/classmap_generator.php` has been removed and is no longer available.
+> Instead, we recommend using [Composer for autoloading](https://getcomposer.org/doc/04-schema.md#autoload).
 
 If you are upgrading from an existing Zend Framework v2 application, please refer to the [migration guide](https://docs.zendframework.com/tutorials/migration/to-v3/application/#scripts) for more detailed information.