[2.2] Updating some URLs from 2.1/master to 2.2
Update introduction.rst
[Reference] [UserPassword] Use the non-deprecated class for Symfony 2.2
[Reference] [UserPassword] Add a note concerning the namespace change
[Reference] [UserPassword] Updated deprecation note
Merge pull request #2297 from michaelperrin/fix/ref-UniquePassword-depr

[Reference] [UserPassword] Use the non-deprecated class for Symfony 2.2
Merge pull request #2294 from janschoenherr/patch-2

Update introduction.rst
Update twig_reference.rst

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

Fixes typo
Merge pull request #2283 from symfony/2.2-update-urls

[2.2] Updating some URLs from 2.1/master to 2.2
[#2283] Updating URL from 2.1 to master on master branch