DOC: add sep argument to read_clipboard signature
- [x] closes #13726


## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14537?src=pr) is 85.26% (diff: 100%)

> Merging [#14537](https://codecov.io/gh/pandas-dev/pandas/pull/14537?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

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 I would not list `delim_whitespace` specifically, as this is actually a equivalent for `sep='\s+'` (that is the reason it is checked for in the code, as it is an alternative way to specify the `sep`).

So would only list `sep` of the two, and explain clearly what the default `sep` values means (you can even say it is equivalent to `delim_whitespace=True`).
 @jorisvandenbossche Thanks for your review. While that makes sense, as issue https://github.com/pandas-dev/pandas/issues/13726 mentions, with both the arguments added, it'll be helpful for argument completion in interactive use. Thoughts?
 > with both the arguments added, it'll be helpful for argument completion in interactive use

The point I was trying to make is that in the current way they are specified, having both arguments is not useful: `sep='\s+'` is the default, and setting `delim_whitespace` to True is equivalent with this, so in practice you never need this. 

Towards the users, I would maybe rather make `delim_whitespace=True` the default, as that is easier to understand what it means compared to `sep='\s+'`. But that is maybe a bit more complex to implement. 
So to conclude, I would just document `sep`, and leave `delim_whitespace` out of the signature.
 @jorisvandenbossche ah, got it. Will remove `delim_whitespace` from the signature.
 @TomAugspurger Can this be merged?
 Looks like the build fails due to this? https://github.com/pypa/pip/issues/4059
 @keshavramaswamy Thanks a lot! (I made a small adjustment before merging, and made the docstring a raw string, which is needed for sphinx to interpret the `\` literally)
