DOC: Document existing functionality of pandas.DataFrame.to_sql() #11886
- [x] Adds documentation for #11886 
- [ ] doc build fails (may be related to #26723)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] **What's new?** Adds documentation to an existing functionality. In fact, the functionality is documented at `pandas.io.sql.to_sql` but it's not visible [here](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_sql.html).
