add a caution block about php-fpm pool

Author: tristanbes

components/process.rst

@@ -135,6 +135,14 @@ are done doing other stuff::
     advantage of the ``kernel.terminate`` event, and run your command **synchronuously**
     inside this event. Be aware that ``kernel.terminate`` is called only if you run ``PHP-FPM``.
 
+.. caution::
+
+    Beware also that if you do that, the said php process won't available to serve
+    any new request until the subprocess is finished, which means you can block your
+    FPM pool quickly if you're not careful enough.
+    That's why it generally way better to not do any fancy thing even after the request is sent
+    but prefer using a job queue.
+
 :method:`Symfony\\Component\\Process\\Process::wait` takes one optional argument:
 a callback that is called repeatedly whilst the process is still running, passing
 in the output and its type::