to_timedelta docs warning on default behaviour
- [x] closes #34633
- [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

Have added clarification to the user guide on the default behaviour of `to_timedelta` when a string is passed.
Hi @mroeschke - I've made this small documentation contribution which appears to work fine but I'm seeing timeout errors in the CI jobs. Do you have an opinion on how to fix these errors or whether this is an issue? No need to worry about the CI failures @TAJD. It looks unrelated to your PR.  Thanks @TAJD 