DOC: include defaults into docs for true_values and false_values

[AlexKirko]

• closes DOC: pandas.read_csv #48487

Motivation

This PR touches only one docstring, so no other checks are necessary. As outlined in #48487, when we call pd.read_csv, we eventually go to _libs/ops.pyx, where there are defaults for true_values and false_values:

    # the defaults
    true_vals = {'True', 'TRUE', 'true'}
    false_vals = {'False', 'FALSE', 'false'}

    if true_values is not None:
        true_vals = true_vals | set(true_values)

    if false_values is not None:
        false_vals = false_vals | set(false_values)

In addition, there appears to be a str.lower() or str.upper() call somewhere, although I can't find it, because trUe is treated as true-like too. We should outline this in the docstring, so that it's not a surprise to our users.

Definition of done

• mention the default values for true_values and false_values in the docstring for pd.read_csv

[AlexKirko]

I don't think a whatsnew entry or anything else is needed for a doc-only PR, but please tell me if I've missed anything.

[AlexKirko]

There are a bunch of errors, but it doesn't look like they have anything to do with the docstring change, and they seem to be cropping up over all the current PRs.

[mroeschke]

Do you know if there are tests for case-insensitive variants of True/False?

[AlexKirko]

That's a great question, it will tell us whether the case insensitivity is intended or unnoticed behavior. I'll take a look today or tomorrow.

[AlexKirko]

@mroeschke I've taken a look, and the closest that I can find is test_boolean_dtype here. There are also some conversion tests in test_na_values.py. But those have a different purpose.

What would your suggestion be?

[mroeschke]

Thanks for checking. I think test_boolean_dtype should cover the additional docstring note you added here.

[mroeschke]

Thanks @AlexKirko