[Upgrading] Recommend to delete cache when upgrading to a major version

Author: javiereguiluz

setup/upgrade_major.rst

@@ -182,6 +182,16 @@ Next, use Composer to download new versions of the libraries:
 
     $ composer update "symfony/*"
 
+It's also recommended to clear the cache after updating to a new major version:
+
+.. code-block:: terminal
+
+    # this deletes the development cache in your local machine
+    $ php bin/console cache:clear
+
+    # also delete the production cache in your local machine
+    $ APP_ENV=prod php bin/console cache:clear
+
 .. include:: /setup/_update_dep_errors.rst.inc
 
 .. include:: /setup/_update_all_packages.rst.inc