[FrameworkBundle] added a note about the classes cache

src/Symfony/Bundle/FrameworkBundle/Command/CacheWarmupCommand.php

@@ -37,6 +37,11 @@ class CacheWarmupCommand extends ContainerAwareCommand
 The <info>%command.name%</info> command warms up the cache.
 
 Before running this command, the cache must be empty.
+
+This command does not generate the classes cache (as when executing this
+command, too many classes that should be part of the cache are already loaded
+in memory). Use <comment>curl</comment> or any other similar tool to warm up
+the classes cache if you want.
 EOF
             )
         ;