[2.3][Form] Cleanup & fix phpdocs

| Q             | A
| ------------- | ---
| Bug fix?      | kinda
| New feature?  | no
| BC breaks?    | no
| Tests pass?   | yes
| License       | MIT

This PR was done mostly cause of reports about invalid/not supported types/variables in phpstorm/scrutinizer-ci, and after I started fixing I noticed more problems in those phpdocs so I have cleanedup them a bit.
Better use: array|FormTypeExtensionInterface[]
http://phpdoc.org/docs/latest/guides/types.html#arrays