minor #20909 [Runtime] Clarify the code flow a little bit more (chx)

This PR was submitted for the 7.3 branch but it was merged into the 6.4 branch instead.

Discussion
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[Runtime] Clarify the code flow a little bit more

Just a few words on the code flow and a potential pitfall.

Commits
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f98e8dc Clarify the code flow a little bit more

Author: javiereguiluz

components/runtime.rst

@@ -36,6 +36,8 @@ So how does this front-controller work? At first, the special
 the component. This file runs the following logic:
 
 #. It instantiates a :class:`Symfony\\Component\\Runtime\\RuntimeInterface`;
+#. The runtime includes the front-controller script -- in this case
+   ``public/index.php`` -- making it run again. Make sure this doesn't cause problems.
 #. The callable (returned by ``public/index.php``) is passed to the Runtime, whose job
    is to resolve the arguments (in this example: ``array $context``);
 #. Then, this callable is called to get the application (``App\Kernel``);