minor #5468 [Cookbook][Templating] Add note about cache warming namespaced twig templates (kbond)

This PR was merged into the 2.3 branch.

Discussion
----------

[Cookbook][Templating] Add note about cache warming namespaced twig templates

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | symfony/symfony#14764
| Applies to    | 2.3
| Fixed tickets | #5391

Commits
-------

cbd86ce add note about cache warming

Author: xabbuh

cookbook/templating/namespaced_paths.rst

@@ -74,6 +74,13 @@ directory:
             );
         ));
 
+.. caution::
+
+    Prior to 2.8, templates in custom namespaces are not pre-compiled by
+    Symfony's cache warmup process. They are compiled on demand. This may
+    cause problems if two simultaneous requests are trying to use the
+    template for the first time.
+
 The registered namespace is called ``foo_bar``, which refers to the
 ``vendor/acme/foo-bar/templates`` directory. Assuming there's a file
 called ``sidebar.twig`` in that directory, you can use it easily: