Improve documentation for subject and object categories

[matentzn]

Solves #256

Resolves [#ISSUE, #ISSUE]

• docs/ have been added/updated if necessary
• make test has been run locally
• tests have been added/updated (if applicable)
• CHANGELOG.md has been updated.

[Description, mentioning at least relevant #ISSUE and how it was addressed. A bulleted list of all changes performed by the PR is is helpful.]

[saubin78]

Thanks, this is much clearer with the category examples in particular.
Yet, there is still something confusing. As you state that this "is documentation for human reviewers", I suggest that the final example be a term label, e.g. "anatomical entity", instead of an identifier "UBERON:0001062".

[matentzn]

@saubin78 thats right, can you check again?

[saubin78]

This looks like a good compromise as it allows using a controlled vocabulary or not, a label or a CURIE. 👍

[matentzn]

Thank you @saubin78