[Process] Document the getLastOutputTime() method.

fixes #12194

Author: OskarStark

components/process.rst

@@ -96,6 +96,12 @@ with a non-zero code)::
         echo $exception->getMessage();
     }
 
+.. tip::
+
+    You can get the last output time in seconds by using the
+    :method:`Symfony\\Component\\Process\\Process::getLastOutputTime` method.
+    This method returns ``null`` if the process wasn't started!
+
 Using Features From the OS Shell
 --------------------------------