Add documentation ArrayDeclarationSniff

I updated this documentation to get an overview for myself about what that monster sniff actually do. 
The order of the violations in the documentation is the same order as in the code.

Please have also a look at this issue which I think its a bug in the sniff: https://pear.php.net/bugs/bug.php?id=20026
You're commented on line with `}`. How this relates to documentation.