Improved the explanation about the "secret" configuration parameter

Author: javiereguiluz

reference/configuration/framework.rst

@@ -60,10 +60,24 @@ secret
 
 **type**: ``string`` **required**
 
-This is a string that should be unique to your application. In practice,
-it's used for generating the CSRF tokens, but it could be used in any other
-context where having a unique string is useful. It becomes the service container
-parameter named ``kernel.secret``.
+This is a string that should be unique to your application and it's commonly used
+to add more entropy to security related operations. Its value should be series of
+characters, numbers and symbols choosen randomly. It's recommended length is
+around 32 characters.
+
+In practice, Symfony uses this value for generating the :ref:`CSRF tokens <forms-csrf>`,
+for encrypting the cookies used in the :doc:`remember me functionality </cookbook/security/remember_me>`
+and for creating signed URIs when using :ref:`ESI (Edge Side Includes) <edge-side-includes>` .
+
+This option becomes the service container parameter named ``kernel.secret``,
+which you can use whenever the application needs a immutable random string
+to add more entropy.
+
+As any other security-related parameter, is a good practice to change this
+value from time to time. However, keep in mind that changing this value will
+invalidate all signed URIs and Remember Me cookies. That's why, after changing
+this value, you should regenerate the application cache, delete the HTTP Cache
+related cache and log out all the application users.
 
 .. _configuration-framework-http_method_override: