ItemAdapter docstrings should mention attrs

[kmike]

Currently they are focused on dict, scrapy.Item and dataclasses, e.g. without having to take the item's implementation (scrapy.Item, dict, dataclass) into account.

[elacuesta]

Right, I think mentioning no specific type is simpler and easier to maintain. Also removed references to pipelines and middlewares, which are too Scrapy-specific. Addressed by 88c4821