DOC: Reindexing behaviour of dataframe column-assignment missing
- [X] xref #39845 (.loc[] portion of issue)
- [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
- [ ] whatsnew entry
