minor #6535 Improved the description of the Twig global variables (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #6535).

Discussion
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Improved the description of the Twig global variables

I was reading this section and I realized that these variables weren't explained but just described with "useless" descriptions. I propose to explain them more deeply.

Commits
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1873b67 Improved the description of the Twig global variables

Author: xabbuh

book/templating.rst

@@ -1202,15 +1202,22 @@ instance which will give you access to some application specific variables
 automatically:
 
 ``app.security``
-    The security context.
+    The :class:`Symfony\\Component\\Security\\Core\\SecurityContext` object or
+    ``null`` if there is none.
 ``app.user``
-    The current user object.
+    The representation of the current user or ``null`` if there is none. The
+    value stored in this variable can be a :class:`Symfony\\Component\\Security\\Core\\User\\UserInterface`
+    object, any other object which implements a ``__toString()`` method or even
+    a regular string.
 ``app.request``
-    The request object.
+    The :class:`Symfony\\Component\\HttpFoundation\\Request` object that represents
+    the current request (depending on your application, this can be a sub-request
+    or a regular request, as explained later).
 ``app.session``
-    The session object.
+    The :class:`Symfony\\Component\\HttpFoundation\\Session\\Session` object that
+    represents the current user's session or ``null`` if there is none.
 ``app.environment``
-    The current environment (dev, prod, etc).
+    The name of the current environment (``dev``, ``prod``, etc).
 ``app.debug``
     True if in debug mode. False otherwise.