Add additional docs for subject and object match fields

[matentzn]

Clarifying the intended use of subject and object match field in the main slot descriptions.

Fixes #413

• 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.

If you are proposing a change to the SSSOM metadata model, you must

• provide a full, working and valid example in examples/
• provide a link to the related GitHub issue in the see_also field of the linkml model
• provide a link to a valid example in the see_also field of the linkml model
• run SSSOM-Py test suite against the updated model

Make it a bit clearer how subject and object_match_field are supposed to be used.

@nichtich does this work for you?

[gouttegd]

Minor nitpicks, but otherwise LGTM.

[matentzn]

@ehartley thank you for the suggestions; @gouttegd sorry for having to ask for another approval, this is mandated after changes to a PR.