minor #11583 Mentioned the deprecation of the threads config option (javiereguiluz)

This PR was merged into the 4.3 branch.

Discussion
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Mentioned the deprecation of the threads config option

This was deprecated in https://github.com/symfony/symfony/pull/31140/files#r276262507

Commits
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730689c Mentioned the deprecation of the threads config option

Author: OskarStark

reference/configuration/security.rst

@@ -240,6 +240,12 @@ encoding algorithm. Also, each algorithm defines different config options:
             ],
         ]);
 
+.. deprecated:: 4.3
+
+    The ``threads`` configuration option was deprecated in Symfony 4.3. No
+    alternative is provided because starting from Symfony 5.0 this value will be
+    hardcoded to ``1`` (one thread).
+
 .. versionadded:: 4.3
 
     The ``sodium`` algorithm was introduced in Symfony 4.3. In previous Symfony