[Docs][C++] "value_counts" kernel doc incorrectly mentions to skip nulls

[jorisvandenbossche]

The "value_counts" kernel indicates that nulls in the input are skipped:

arrow/cpp/src/arrow/compute/kernels/vector_hash.cc

Lines 748 to 753 in 431785f

	const FunctionDoc value_counts_doc(
	"Compute counts of unique elements",
	("For each distinct value, compute the number of times it occurs in the array.\n"
	"The result is returned as an array of `struct<input type, int64>`.\n"
	"Nulls in the input are ignored."),
	{"array"});

But that's not actually the case. Nulls are also counted and included in the output:

In [1]: import pyarrow.compute as pc

In [2]: pc.value_counts([1, 2, 2, None, None])
Out[2]: 
<pyarrow.lib.StructArray object at 0x7fb88e1c6f20>
-- is_valid: all not null
-- child 0 type: int64
  [
    1,
    2,
    null
  ]
-- child 1 type: int64
  [
    1,
    2,
    2
  ]

[jorisvandenbossche]

Same is true for unique:

In [3]: pc.unique([1, 2, 2, None, None])
Out[3]: 
<pyarrow.lib.Int64Array object at 0x7fb88e1c75e0>
[
  1,
  2,
  null
]

And to show it's just the docs, we have various tests that cover this.