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
[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
minor #3739 [book] [forms] misc. fixes and tweaks (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

[book] [forms] misc. fixes and tweaks

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

Commits
-------

1583f3b [book] [forms] minor fixes and rewordings
d4336b7 [book] [forms] misc. fixes and tweaks
minor #3736 [book] Misc. routing fixes (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

[book] Misc. routing fixes

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

Commits
-------

7500435 [book] [routing] minor rewording and fixed some sample code
26f9e3b [book] [routing] fixed a note that wasn't properly updated
355cd5b [book] [routing] reworded the note about generating URLs in console
da8b46f [book] [routing] used the American term "bidirectional" instead of the British bi-directional
bug #3741 correct RuntimeException reference (shieldo)

This PR was merged into the 2.3 branch.

Discussion
----------

correct RuntimeException reference

Commits
-------

5d2e41c correct RuntimeException reference
Fixed event listeners priority
Fixed event listeners priority (typos after review)
Replaced the priority paragraph with a note and a table
Fixed title underline
Updated title for the core cache warmers
Fixed title level consistency
feature #3719 Fixed event listeners priority (tony-co)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3719).

Discussion
----------

Fixed event listeners priority

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
-------

47d4d4f Fixed title level consistency
1dbfd29 Updated title for the core cache warmers
a9d07a5 Fixed title underline
815f531 Replaced the priority paragraph with a note and a table
db2c002 Fixed event listeners priority (typos after review)
13b5645 Fixed event listeners priority
add serializer set callback documentation