Enhanced Firewall Restrictions docs

Squashed commits:
[2081fc4] Enhanced Firewall Restrictions docs for PR symfony/symfony#10404
[8f065bc] Add redirect to new page
[b42e80f] fixed typos
[cc6b07d] tiny update
After backporting something from master (2.5), removing the 2.5-specific features
Minor tweaks related to #3368
Minor tweaks after merging #3644 by @stof and @xabbuh
Fixing syntax error
[book] [validation] fixed minor typos
[book] [testing] fixed the path of the phpunit.xml file

Sometimes is referenced as phpunit.xml and other times as
app/phpunit.xml I think that we should always use the explicit
app/phpunit.xml path, except when explaining the phpunit.xml
file in general.
[book] [controller] fixed the code of a session sample code

This code was updated in the past to use the $request controller
injection and the resulting code was a bit confusing. When you get
the attribute set by another controller, it's better to use a
different attribute name, to make it clear that it wasn't set at
this controller.
[book] [routing] used the American term "bidirectional"
instead of the British bi-directional
[book] [routing] reworded the note about generating URLs in console
[book] [routing] fixed a note that wasn't properly updated

Previously, the code used the generate() method of the service and
the note explained the generateUrl() method of the base controller.
Then, the example was updated to use the generateUrl(), but the
note kept explaining the generateUrl(). This change is just about
explaining in the note the alterantive generate() method of the service.
[book] [forms] misc. fixes and tweaks
[book] [forms] minor fixes and rewordings
[book] [routing] minor rewording and fixed some sample code
correct RuntimeException reference
Update security.rst

I'm not a native speaker but I think that it sounds better by adding that before the word `start`.
Improved the sentence
minor #3636 Update security.rst (nomack84)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3636).

Discussion
----------

Update security.rst

I'm not a native speaker but I think that it reads better now.

Commits
-------

1c2abab Improved the sentence
c8c093a Update security.rst
minor #3725 Minor tweaks related to #3368 (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
----------

Minor tweaks related to #3368

Hi guys!

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3

Commits
-------

544a0f5 Minor tweaks related to #3368
minor #3726 Minor tweaks after merging #3644 by @stof and @xabbuh (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
----------

Minor tweaks after merging #3644 by @stof and @xabbuh

Hi guys!

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | 2.3+
| Fixed tickets | #3644

Cheers!

Commits
-------

cac5b03 Fixing syntax error
f83ff09 Minor tweaks after merging #3644 by @stof and @xabbuh