Add explanation how to make the Process survive a response

Fixes #7151

It confuses user that could be tempted to use the Asynchronous Process feature to achieve long running task after the Response is sent.

The result is the following :
from no code execution (if you send a Response really fast)
to partial code execution

Author: tristanbes

components/process.rst

@@ -124,6 +124,16 @@ are done doing other stuff::
     which means that your code will halt at this line until the external
     process is completed.
 
+.. note::
+    If a `Response` is sent **before** what `Process` is running had a chance to complete,
+    the server process will be killed (depending on your OS). It means that your task
+    will be stopped right away.
+    Running an asynchronous process is not the same than running a processing surviving yourselves.
+
+    If you want your process to survive the request/response cycle, you could take
+    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`.
+
 :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::