Mention PCNTL extension installation

Author: alexandre-daubois

components/console/events.rst

@@ -152,6 +152,8 @@ Listeners receive a
     It is then dispatched just after the ``ConsoleEvents::ERROR`` event.
     The exit code received in this case is the exception code.
 
+.. _console_signal-event:
+
 The ``ConsoleEvents::SIGNAL`` Event
 -----------------------------------
 
@@ -184,7 +186,8 @@ Listeners receive a
 .. tip::
 
     All the available signals (``SIGINT``, ``SIGQUIT``, etc.) are defined as
-    `constants of the PCNTL PHP extension`_.
+    `constants of the PCNTL PHP extension`_. The extension has to be installed
+    for these constants to be available.
 
 If you use the Console component inside a Symfony application, commands can
 handle signals themselves. To do so, implement the

components/lock.rst

@@ -269,6 +269,11 @@ Lock will be released automatically as soon as one process finishes::
     }
     // ...
 
+.. note::
+
+    In order for the above example to work, the `PCNTL`_ extension must be
+    installed.
+
 To disable this behavior, set the ``autoRelease`` argument of
 ``LockFactory::createLock()`` to ``false``. That will make the lock acquired
 for 3600 seconds or until ``Lock::release()`` is called::
@@ -1074,3 +1079,4 @@ are still running.
 .. _`ZooKeeper`: https://zookeeper.apache.org/
 .. _`readers-writer lock`: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
 .. _`priority policy`: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Priority_policies
+.. _`PCNTL`: https://www.php.net/manual/book.pcntl.php