feature #6539 Documented the "autoescape" TwigBundle config option (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #6539).

Discussion
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Documented the "autoescape" TwigBundle config option

This option was missing after the recent doc update.

Commits
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ea79276 Documented the "autoescape" TwigBundle config option

Author: wouterj

reference/configuration/twig.rst

@@ -112,6 +112,35 @@ If ``true``, whenever a template is rendered, Symfony checks first if its source
 code has changed since it was compiled. If it has changed, the template is
 compiled again automatically.
 
+autoescape
+~~~~~~~~~~
+
+**type**: ``boolean`` or ``string`` **default**: ``'filename'``
+
+If set to ``true``, all template contents are escaped for HTML. If set to
+``false``, automatic escaping is disabled (you can still escape each content
+individually in the templates).
+
+.. caution::
+
+    Setting this option to ``false`` is dangerous and it will make your
+    application vulnerable to XSS exploits because most third-party bundles
+    assume that auto-escaping is enabled and they don't escape contents
+    themselves.
+
+If set to a string, the template contents are escaped using the strategy with
+that name. Allowed values are ``html``, ``js``, ``css``, ``url``, ``html_attr``
+and ``filename``. The default value is ``filename`` and it escapes contents
+according to the filename extension (e.g. it uses ``html`` for ``*.html.twig``
+templates and ``js`` for ``*.js.html`` templates).
+
+.. tip::
+
+    See :ref:`config-twig-autoescape-service` and :ref:`config-twig-autoescape-service-method`
+    to define your own escaping strategy.
+
+.. _config-twig-autoescape-service:
+
 autoescape_service
 ~~~~~~~~~~~~~~~~~~
 
@@ -125,6 +154,8 @@ for HTML and the contents of ``*.js.twig`` are escaped for JavaScript.
 This option allows to define the Symfony service which will be used to determine
 the default escaping applied to the template.
 
+.. _config-twig-autoescape-service-method:
+
 autoescape_service_method
 ~~~~~~~~~~~~~~~~~~~~~~~~~