docstrings: typos, clarity
Minor changes only in docstrings: capitalisation, hyphenation, "indexes" -> "indices", typos.

- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Apologies for not testing, as the changes are minor and only affect docstrings.
No `whatsnew` entry either because minor.