[OptionsResolver] Adjusted the OptionsResolver documentation to describe the 2.6 API
Added missing mailer class and use statements
Removed duplicate use statements
[#4003] A few more form_themes config changes
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
Replace addViolationAt (deprecated) by buildViolation
Update Callback.rst

"Symfony\Component\Validator\ExecutionContextInterface" namespace is deprecated, new namespace should be "Symfony\Component\Validator\Context\ExecutionContextInterface"
Don't use deprecated functions in Callback.rst

The addViolationAt function is marked as deprecated and should be replaced by buildViolation according to the Symfony update guides.
Update custom_contraint.rst to meet the new 2.5 api
added a versionadded comment to Callback.rst
Adding details about the 2.4 API as comments
Adding a section about keeping BC in a re-usable bundle
Fixes thanks to @xabbuh
Minor tweaks and a missing location thanks to xabbuh and WouterJ
[#4233][#4094] Making validateValue and validate changes
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
Merge pull request #1 from peterrehm/patch-9

Added mailer class and use statements
validate `null` (Expression constraint in 2.6)

Since Symfony 2.6, the Expression constraint doesn't skip validating
`null` values.