[documentation] Added description of Role Handler

There was a lot of doc about ACL, but (nearly) nothing about the role handler. So here is some precisions about it.
Commented about access_strategy
Added support for setting group translation_domain

- Ability to set translation_domain on a form group
- Translation domain cascades to group fields
- Added unit test for FormMapper
- Extended AdminInterface to have [get|set]FormGroups
Merge pull request #1359 from romaricdrigon/patch-1

[documentation] Added description of Role Handler
Updated doc
Merge pull request #1378 from dantleech/ext_trans_domain_cascade

Added support for setting group translation_domain
Merge branch 'sonata/master'