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
support Varnish in configuration blocks
Updating library/bundle installation docs to use the new composer require (no version) functionality
Fixing bad link
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 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
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.
first import of the "Official Best Practices" book
Fixed some technical problems as suggested by Wouter
Added an index directive to the first chapter