Language improvements
Fixed safety statements
Added that adding custom properties is not safe
Fix issue #3442

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets | #3442
Documented deprecation of the apache router.
Fix error in namespace when use TokenInterface

Change the use statement of TokenInterface to the correct namespace (missing Token folder)
[#3407] Adding a missing section via tip from @WouterJ
bug #3421 [Contributing] Cleaning the "contributing patch" page a bit (lemoinem)

This PR was merged into the 2.4 branch.

Discussion
----------

[Contributing] Cleaning the "contributing patch" page a bit

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

I just mentioned the fact that `push -f` is required when rebasing and explicit that the "2.2 when bugfixing" is a follow up on the previous example (Not necessarily explicit at first).

I also worked my way through the all document to replace wrong single back-tick occurrences by double backticks.

Finally, I converted the mentions of PSR-1 and PSR-2 to links toward the respective PSR.

PS: as per @WouterJ request (https://github.com/symfony/symfony-docs/pull/3421#issuecomment-31585092), I also replaced mentions of 2.2 by 2.3

Commits
-------

c0de1cb [Contributing] Replace HEAD~3 by upstream/master
732758b [Contributing] Fix "When you finish" => "When you are finished"
b25ec96 [Contributing] Replace mentions of 2.2 for 2.3
70ed645 [Contributing] Add PSR-1 and 2 links
6796ba8 [Contributing] Fix single vs double backtick issues
b35e62b [Contributing] Clarify the rebase operation
[Contributing] Clarify the rebase operation

I just mentioned the fact that `push -f` is usually required when rebasing and explicit that the "2.2 when bugfixing" is a follow up on the previous example (Not necessarily explicit at first).
[Contributing] Fix single vs double backtick issues
[Contributing] Add PSR-1 and 2 links
[Contributing] Replace mentions of 2.2 for 2.3

As per WouterJ request: https://github.com/symfony/symfony-docs/pull/3421#issuecomment-31585092
[Contributing] Fix "When you finish" => "When you are finished"

As per @xabbuh suggestion: https://github.com/symfony/symfony-docs/pull/3421#discussion_r8661716
[Contributing] Replace HEAD~3 by upstream/master

As per @cordoval suggestion: https://github.com/symfony/symfony-docs/pull/3421#discussion_r8670868
Merge branch '2.3' into 2.4
minor #3422 [Cookbook][Custom Authentication Provider] add a note of warning for when forbidding anonymous users (cordoval)

This PR was merged into the 2.3 branch.

Discussion
----------

[Cookbook][Custom Authentication Provider] add a note of warning for when forbidding anonymous users

| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | yes                 |
| New docs?     | no                  |
| Applies to    | 2.3+                |
| Fixed tickets | #3212               |
| License       | CC-ASA 3.0 Unported |

 Sent using [Gush](https://github.com/cordoval/gush)

Commits
-------

244d756 add note when forbidding access to anonymous users on custom authentication provider
bug #3423 [Session Configuration] add clarifying notes on session save handler proxies (cordoval)

This PR was merged into the 2.3 branch.

Discussion
----------

[Session Configuration] add clarifying notes on session save handler proxies

| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | yes                 |
| New docs?     | no                  |
| Applies to    | 2.3+                |
| Fixed tickets | #2889               |
| License       | CC-ASA 3.0 Unported |

 Sent using [Gush](https://github.com/cordoval/gush)

Commits
-------

771b862 address comments from @WouterJ
2888d00 answer some questions and clarify the best chapter imo from the symfony documentation
feature #3428 Translation - Added info about JsonFileLoader added in 2.4 (singles)

This PR was merged into the 2.4 branch.

Discussion
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Translation - Added info about JsonFileLoader added in 2.4

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8534)
| Applies to    | 2.4
| Fixed tickets | n/a

Commits
-------

adf678b Added info about JsonFileLoader added in 2.4
bug #3432 [Reference][Form Types] Add "max_length" option in form type (nykopol)

This PR was merged into the 2.3 branch.

Discussion
----------

[Reference][Form Types] Add "max_length" option in form type

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #3410 (task Form -> max_length)

Commits
-------

b53a227 fix last typo on max length option
d6800d3 fix typo in previous commit
391fe30 fix typo in previous commit
2156545 add max_length to form field type
Applied suggestions from Ryan