DOC: improve IO & General Functions API reference
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them


* Improve formatting of reference pages.
* Improve some doc summary lines so they are clearer in the reference pages.

@meeseeksdev backport 1.4.x Something went wrong ... Please have a look at my logs.