[TwigBundle] Describe the new behaviour of twig.cache config

Signed-off-by: Quentin Devos <4972091+Okhoshi@users.noreply.github.com>

Author: okhoshi

reference/configuration/twig.rst

@@ -71,7 +71,7 @@ application harder to maintain.
 cache
 ~~~~~
 
-**type**: ``string`` | ``false`` **default**: ``%kernel.cache_dir%/twig``
+**type**: ``string`` | ``boolean`` **default**: ``true``
 
 Before using the Twig templates to render some contents, they are compiled into
 regular PHP code. Compilation is a costly process, so the result is cached in
@@ -82,6 +82,12 @@ is not recommended; not even in the ``dev`` environment, because the
 ``auto_reload`` option ensures that cached templates which have changed get
 compiled again.
 
+Set this option to the path where the cache should be stored. If set to
+``true``, the cache is stored in ``%kernel.cache_dir%/twig``. However, if the
+``auto_reload`` option is not enabled and ``%kernel.build_dir`` is configured
+to be different than ``%kernel.cache_dir``, the cache is stored in
+``%kernel.build_dir%/twig``.
+
 charset
 ~~~~~~~