$document -> $object in some use cases

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #110 
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/438 needs to be refactored same way

Just refactored some vars named `$document` to `$object` cause in that use cases all kinds of objects/entities/documents are useable.