Update model card's eval results documentation (#1144)
* rewrite evaluation results doc
* Update docs/hub/model-cards.md
Co-authored-by: Julien Chaumond <julien@huggingface.co>
* source is optional but values are required if any
* make source name optional
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Co-authored-by: Julien Chaumond <julien@huggingface.co>
