Update guidance for unsigned integer data types (#340)

Author: kgryte

spec/API_specification/statistical_functions.md

@@ -134,7 +134,9 @@ For an input array `x`, let `N` equal the number of elements over which to compu
     -   data type of the returned array. If `None`,
 
         -   if the default data type corresponding to the data type "kind" (integer or floating-point) of `x` has a smaller range of values than the data type of `x` (e.g., `x` has data type `int64` and the default data type is `int32`, or `x` has data type `uint64` and the default data type is `int64`), the returned array must have the same data type as `x`.
-        -   otherwise, the returned array must have the default data type corresponding to the data type "kind" (integer or floating-point) of `x`.
+        -   if `x` has a floating-point data type, the returned array must have the default floating-point data type.
+        -   if `x` has a signed integer data type (e.g., `int16`), the returned array must have the default integer data type.
+        -   if `x` has an unsigned integer data type (e.g., `uint16`), the returned array must have an unsigned integer data type having the same number of bits as the default integer data type (e.g., if the default integer data type is `int32`, the returned array must have a `uint32` data type).
 
         If the data type (either specified or resolved) differs from the data type of `x`, the input array should be cast to the specified data type before computing the product. Default: `None`.
 
@@ -217,7 +219,9 @@ For an input array `x`, let `N` equal the number of elements over which to compu
     -   data type of the returned array. If `None`,
     
         -   if the default data type corresponding to the data type "kind" (integer or floating-point) of `x` has a smaller range of values than the data type of `x` (e.g., `x` has data type `int64` and the default data type is `int32`, or `x` has data type `uint64` and the default data type is `int64`), the returned array must have the same data type as `x`.
-        -   otherwise, the returned array must have the default data type corresponding to the data type "kind" (integer or floating-point) of `x`.
+        -   if `x` has a floating-point data type, the returned array must have the default floating-point data type.
+        -   if `x` has a signed integer data type (e.g., `int16`), the returned array must have the default integer data type.
+        -   if `x` has an unsigned integer data type (e.g., `uint16`), the returned array must have an unsigned integer data type having the same number of bits as the default integer data type (e.g., if the default integer data type is `int32`, the returned array must have a `uint32` data type).
 
         If the data type (either specified or resolved) differs from the data type of `x`, the input array should be cast to the specified data type before computing the sum. Default: `None`.