minor #5630 Add a caution about logout when using http-basic authenticated firewall (rmed19)

This PR was merged into the 2.3 branch.

Discussion
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Add a caution about logout when using http-basic authenticated firewall

Hi,
I found many developers on stackoverflow asking how to logout from http_basic auth in Symfony2.
So I added a caution about it.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | +2.3

Commits
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3a0bd9d Add a caution about logout when using http-basic authenticated firewall

Author: xabbuh

book/security.rst

@@ -1135,6 +1135,13 @@ is defined by the ``target`` parameter above (e.g. the ``homepage``).
     :class:`Symfony\\Component\\Security\\Http\\Logout\\LogoutSuccessHandlerInterface`.
     See :doc:`Security Configuration Reference </reference/configuration/security>`.
 
+.. caution::
+
+    Notice that when using http-basic authenticated firewall there is no real 
+    way to log out : the only way to 'logout' is to have the browser stop sending your name and password 
+    on every request. Clearing your browser cache, restarting your browser usually helps, and some web developer
+    tools might be helpful here.
+
 .. _`security-encoding-password`:
 
 Dynamically Encoding a Password