DOC: add examples to insert and update generally
This commit mainly adds examples on usage. This commit also fills in
information suggested by `validate_docstrings.py` script.

Ran

```
python scripts/validate_docstrings.py pandas.DataFrame.insert
```

to check docs and added in suggested information (e.g., See Also section).

- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
