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
feature #4233 2.5 Validation API changes (nicolassing, lashae, Rootie, weaverryan)

This PR was merged into the 2.5 branch.

Discussion
----------

2.5 Validation API changes

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.5
| Fixed tickets | #4094

Hi guys!

This takes the work in #4056, #4105 and #4161 and extends it based on some feedback. Basically, the whole validation stuff is quite difficult, because:

A) We want to show the non-deprecated methods so people use the new stuff
B) We don't want to confuse users on the 2.4 API (even though this number of users should be very small, as it would require you to have started a project in the past and be using 5.3.9 and lower https://github.com/symfony/symfony-docs/pull/4105#issuecomment-51594463)

This solution is to show the new way, but always show the old way in comments. This would check of A and B in #4094.

Thanks!

Commits
-------

9874d8e [#4233][#4094] Making validateValue and validate changes
94fc520 Minor tweaks and a missing location thanks to xabbuh and WouterJ
f97ba7a Fixes thanks to @xabbuh
279d8d6 Adding a section about keeping BC in a re-usable bundle
280440e Adding details about the 2.4 API as comments
e658b56 added a versionadded comment to Callback.rst
70c5ca1 Update custom_contraint.rst to meet the new 2.5 api
5dfe499 Don't use deprecated functions in Callback.rst
f4380ed Update Callback.rst
042dcf9 Replace addViolationAt (deprecated) by buildViolation