Add many examples to schema and update some descriptions

[matentzn]

See #392

this PR adds examples to most slots related to mappings, and updates some descriptions.

• [ ] 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

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