MINOR: Update null values documentation in csv.rst

[Aiee]

Rationale for this change

Explicitly showing the default null spellings in the document would be convenient.
Source:

arrow/cpp/src/arrow/csv/options.cc

Lines 38 to 47 in 5eaf553

	ConvertOptions ConvertOptions::Defaults() {
	auto options = ConvertOptions();
	// Same default null / true / false spellings as in Pandas.
	options.null_values = {"", "#N/A", "#N/A N/A", "#NA", "-1.#IND", "-1.#QNAN",
	"-NaN", "-nan", "1.#IND", "1.#QNAN", "N/A", "NA",
	"NULL", "NaN", "n/a", "nan", "null"};
	options.true_values = {"1", "True", "TRUE", "true"};
	options.false_values = {"0", "False", "FALSE", "false"};
	return options;
	}

What changes are included in this PR?

Only the document.

Are these changes tested?

No need to test.

Are there any user-facing changes?

No.

[github-actions]

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[raulcd]

@github-actions crossbow submit preview-docs

[github-actions]

Revision: 7945ea3

Submitted crossbow builds: ursacomputing/crossbow @ actions-d7b30c40c1

Task	Status
preview-docs

[raulcd]

Thanks for the PR, it feels too specific to me and prone to getting outdated soon if we end up doing any other spelling. I am not sure this is something we want to add to the documentation.
@AlenkaF @thisisnic thoughts?
In any case the linter is failing because those require double backticks ``""`` instead of single `""`
See rendered docs not showing correctly here:
http://crossbow.voltrondata.com/pr_docs/48048/cpp/csv.html#nulls

[Aiee]

Thanks for the PR, it feels too specific to me and prone to getting outdated soon if we end up doing any other spelling. I am not sure this is something we want to add to the documentation. @AlenkaF @thisisnic thoughts? In any case the linter is failing because those require double backticks "" instead of single "" See rendered docs not showing correctly here: http://crossbow.voltrondata.com/pr_docs/48048/cpp/csv.html#nulls

Hi, thanks for the reply. My initial motivation is that I found it was hard to find the specific null spelling (also for true_values and `false_values') without diving into the source code. I thought it might be helpful to state it in the document since these values haven't been changed for a long time. But sure, it adds cost for future maintenance.

[AlenkaF]

It is a bit of a paint to find relevant information for this specific case.

What if we update the header file here:

arrow/cpp/src/arrow/csv/options.h

Lines 130 to 132 in 5eaf553

	/// Create conversion options with default values, including conventional
	/// values for `null_values`, `true_values` and `false_values`
	static ConvertOptions Defaults();

with additional information about what are "default null values" (plus others) and then link the API docs section here:
https://arrow.apache.org/docs/cpp/api/formats.html#_CPPv4N5arrow3csv14ConvertOptions8DefaultsEv

to the Nulls section in the User Guide?
http://crossbow.voltrondata.com/pr_docs/48048/cpp/csv.html#nulls

Adding such a specific list directly to the docs does look like a possible future issue with outdated information.

[Aiee]

It is a bit of a paint to find relevant information for this specific case.

What if we update the header file here:

arrow/cpp/src/arrow/csv/options.h

Lines 130 to 132 in 5eaf553

	/// Create conversion options with default values, including conventional
	/// values for `null_values`, `true_values` and `false_values`
	static ConvertOptions Defaults();

with additional information about what are "default null values" (plus others) and then link the API docs section here: arrow.apache.org/docs/cpp/api/formats.html#_CPPv4N5arrow3csv14ConvertOptions8DefaultsEv

to the Nulls section in the User Guide? http://crossbow.voltrondata.com/pr_docs/48048/cpp/csv.html#nulls

Adding such a specific list directly to the docs does look like a possible future issue with outdated information.

Hi, I've reverted the doc change and update the header file as suggested. Please take a look.

[AlenkaF]

We can not link to the preview docs. Here, in the header file, we can actually add the list of the conventional null values. Then this section can be linked from the csv.html.