Refine Body Attribute Guidelines

[dobbersc]

From #138 (comment) we noticed that the attribute guidelines the the body are too loose. We need to add that the body is only constructed from the article's text.

There were occurrences with CNBC and The Intercept where we can parse a summary or description from the article's meta. Since it would be unfortunate to not parse such data, how do we proceed? One way would be to add another attribute along the lines of "summary" or "description".