minor symfony#8978 This doc doesn't apply to version 3.4 (Romain)

This PR was merged into the 3.4 branch.

Discussion
----------

This doc doesn't apply to version 3.4

This documentation doesn't seem to apply to version 3.4.
I tried to use it without any success. I think that we should display  a disclaimer since v3.4 doesn't use the bundle concept anymore.

Commits
-------

7ecbe69 This doc doesn't apply to version 3.4

Author: javiereguiluz

bundles.rst

@@ -6,6 +6,12 @@
 The Bundle System
 =================
 
+.. caution::
+
+    In Symfony versions prior to 3.4, it was recommended to organize your own
+    application code using bundles. This is no longer recommended and bundles
+    should only be used to share code and features between multiple applications.
+
 A bundle is similar to a plugin in other software, but even better. The key
 difference is that *everything* is a bundle in Symfony, including both the
 core framework functionality and the code written for your application.