DOC: Fix to_latex docstring.
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Fix the DataFrame.to_latex docstring to match `scripts/validate_docstrings.py` as explained in #22459  and add an example.

The docstring was previously in a variable that was only used in to_latex. I put it in the method docstring instead. The `@Substitution` wasn't matching anything, I suspect this dates back to the common docstring in `io/formats/format.py`.
