minor symfony#11206 Add missing security guard sub-component (jfhovinne)

This PR was squashed before being merged into the 3.4 branch (closes symfony#11206).

Discussion
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Add missing security guard sub-component

Introduction says "The Security component is divided into four smaller sub-components" while only three are listed.
Added security-guard.

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83cb4b8 Add missing security guard sub-component

Author: javiereguiluz

components/security.rst

@@ -36,6 +36,10 @@ used separately:
 ``symfony/security-csrf``
     It provides protection against `CSRF attacks`_.
 
+``symfony/security-guard``
+    It brings many layers of authentication together, allowing the creation
+    of complex authentication systems.
+
 .. seealso::
 
     This article explains how to use the Security features as an independent