BUG: excelwriter engine_kwargs fix
- [x] closes #29095
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them - *I cannot find the linked page*
- [ ] whatsnew entry


The `engine_kwargs` argument was not passed properly and this was not tested. Also added some documentation.
Hello @Joeperdefloep! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-07-12 21:54:21 UTC We would really love to have this fix in the next version of pandas, since we have to mitigate XLSX injections. Is there something I could help here to achieve this? @Joeperdefloep can you merge master This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @Joeperdefloep do you still want to work on this?  Thanks for the PR, but it appears to have gone stale. Let us know if you're still interested in working on this and we can reopen. Closing. @mattelacchiato - any interest in picking this up? Heyy, sorry I was on holidays a bit so that's why it stopped and went stale...

Also there was a weird thing that there was a merge conflict, where my test was changed to another one. That's why this PR doesn't mention it.

I will be at my computer tomorrow and hopefully will come back to this this week :)