Address review comments

Author: pitrou

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

@@ -112,6 +112,7 @@ struct Winsorize {
         CallFunction("quantile", {input}, &quantile_options, ctx->exec_context()));
     auto quantile_array = quantile.array_as<ArrayType>();
     DCHECK_EQ(quantile_array->length(), 2);
+    // The quantile function outputs either all nulls or no nulls at all.
     if (quantile_array->null_count() == 2) {
       return std::nullopt;
     }

docs/source/cpp/compute.rst

@@ -1728,8 +1728,8 @@ overflow is detected.
 
 * \(2) :member:`CumulativeOptions::start` is ignored.
 
-Other numeric functions
-~~~~~~~~~~~~~~~~~~~~~~~
+Statistical functions
+~~~~~~~~~~~~~~~~~~~~~
 
 +-------------------------+-------+-------------+-------------+--------------------------------+-----------+
 | Function name           | Arity | Input types | Output type | Options class                  | Notes     |