minor #10984 Added a brief mention about upgrading custom bundle config to Flex (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
----------

Added a brief mention about upgrading custom bundle config to Flex

Fixes #10802.

Commits
-------

55a6f5c Added a brief mention about upgrading custom bundle config to Flex

Author: javiereguiluz

setup/flex.rst

@@ -169,6 +169,8 @@ manual steps:
 
 #. Move the original parameters defined in ``app/config/parameters.*.yml`` to
    the new ``config/services.yaml`` and ``.env`` files depending on your needs.
+   If you have defined :doc:`custom config options in your bundles </bundles/configuration>`
+   move them to the new ``config/services.yaml`` and ``.env`` files.
 
 #. Move the original source code from ``src/{App,...}Bundle/`` to ``src/`` and
    update the namespaces of every PHP file to be ``App\...`` (advanced IDEs can do