Make a small grammatical adjustment

The original form of the phrase - «The other option is to specifically
checking if a session has expired after the session is started.» - is
not entirely grammatically correct, as the infinitive form of «to check»
shall normally be used, or the gerund form, which is the form used in
this commit.
[Cookbook][External Parameters] Enhance description of environment variables
Update security.rst
minor #4285 Update security.rst (placid2000)

This PR was submitted for the 2.5 branch but it was merged into the 2.4 branch instead (closes #4285).

Discussion
----------

Update security.rst

Hotfix: allow_if option might been added afterwards and seems to miss in the description above the list

Commits
-------

6a0551c Update security.rst
support Varnish in configuration blocks
Improve readability
minor #4306 Improve readability (ifdattic)

This PR was merged into the 2.3 branch.

Discussion
----------

Improve readability

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

Commits
-------

a715baf Improve readability
minor #4287 support Varnish in configuration blocks (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

support Varnish in configuration blocks

To be able to use them, fabpot/sphinx-php#20 has to be merged first. After that, the submodule reference has to be updated as well with this pull request.

@dbu @thierrymarianne Is Ruby the language to choose to get proper syntax highlighting?

Commits
-------

f2bf980 support Varnish in configuration blocks
Update choice.rst

Add missing parenthesis
bug #4315 Update choice.rst (odolbeau)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4315).

Discussion
----------

Update choice.rst

Add missing parenthesis

Commits
-------

d014607 Update choice.rst
Remove horizontal scrollbar
Remove horizontal scrollbar and change event name to follow conventions
minor #4317 Remove horizontal scrollbar and change event name to follow conventions (ifdattic)

This PR was merged into the 2.3 branch.

Discussion
----------

Remove horizontal scrollbar and change event name to follow conventions

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

In my opinion the event name should follow the naming conventions http://symfony.com/doc/current/components/event_dispatcher/introduction.html#naming-conventions

Commits
-------

7fe8a36 Remove horizontal scrollbar and change event name to follow conventions
minor #4316 Remove horizontal scrollbar (ifdattic)

This PR was merged into the 2.4 branch.

Discussion
----------

Remove horizontal scrollbar

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets |

Commits
-------

53638a6 Remove horizontal scrollbar
Correct a typo: remove unnecessary "the" word.
Correct a typo: replace "then" by "the".
minor #4318 [Cookbook][Bundles] Correct a typo: remove unnecessary "the" word (fabschurt)

This PR was merged into the 2.3 branch.

Discussion
----------

[Cookbook][Bundles] Correct a typo: remove unnecessary "the" word

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

Commits
-------

0d8e3c2 Correct a typo: remove unnecessary "the" word.
minor #4322 [Components][DependencyInjection] Correct a typo: replace "then" by "the" (fabschurt)

This PR was merged into the 2.3 branch.

Discussion
----------

[Components][DependencyInjection] Correct a typo: replace "then" by "the"

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

Commits
-------

10776bb Correct a typo: replace "then" by "the".
Correct a typo.

Removed uncorrect "load use" phrase part, and replaced it with "you can
use", which I guess conveys the original meaning of the phrase better.
Merge remote-tracking branch 'origin/2.4' into 2.5

* origin/2.4:
  Remove horizontal scrollbar
  minor #4285 Update security.rst (placid2000)
  Update security.rst