DOC: fix closed and dtype arguments of TimedeltaIndex
- [X] closes #49153 
- ~~[Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature~~
- [X] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
-  ~~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~~
- ~~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~~

1. **closed**: removed unused argument (xref #49167). It was already deprecated in version 0.24.0. (See https://github.com/TomAugspurger/pandas/blob/4d0fce26a87f7c6d74b77743fbcf96dd9c3cc338/pandas/core/indexes/timedeltas.py#L82)

2. **dtype**: add dtype parameter in docstring and add warning as indicated in [TimedeltaArray](https://pandas.pydata.org/docs/dev/reference/api/pandas.arrays.TimedeltaArray.html), since the argument is used to construct a TimedeltaArray.
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks @abonte for the PR. closing as stale but please comment if you want to continue and will re-open.