minor #15049 [Lock] Added paragraph about auto release (Nyholm)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

[Lock] Added paragraph about auto release

This will fix symfony/symfony#40325

@twisted1919 is this paragraph helpful? Should I add something more?

Commits
-------

84ce746 [Lock] Added paragraph about auto release

Author: javiereguiluz

components/lock.rst

@@ -200,6 +200,38 @@ This component also provides two useful methods related to expiring locks:
 ``getRemainingLifetime()`` (which returns ``null`` or a ``float``
 as seconds) and ``isExpired()`` (which returns a boolean).
 
+Automatically Releasing The Lock
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A Lock will be automatically released when the Lock object is destructed. This is
+an implementation detail that will be important when Locks are shared between processes.
+In the example below, the ``pcntl_fork()`` will create two processes and the Lock
+will be released automatically as soon as one process finishes::
+
+    // ...
+    $lock = $factory->createLock('report-generation', 3600);
+    if (!$lock->acquire()) {
+        return;
+    }
+
+    $pid = pcntl_fork();
+    if ($pid == -1) {
+        // Could not fork
+        exit(1);
+    } elseif ($pid) {
+        // Parent process
+        sleep(30);
+    } else {
+        // Child process
+        echo 'The lock will be released now.'
+        exit(0);
+    }
+    // ...
+
+To disable this behavior, one needs to set the 3rd argument to the ``LockFactory::createLock()``
+to false. That will make the Lock acquired for 3600 seconds or until ``Lock::release()``
+is called.
+
 The Owner of The Lock
 ---------------------