Add many examples to schema and update some descriptions (#394)

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](https://github.com/mapping-commons/sssom/blob/master/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.]