Add a result_type function to the API specification

Identified as needed in data-apisgh-14, also came up in data-apisgh-43 and data-apisgh-91.

Author: rgommers

spec/API_specification/utility_functions.md

@@ -64,3 +64,27 @@ Tests whether any input array element evaluates to `True` along a specified axis
 -   **out**: _<array>_
 
     -   if a logical OR reduction was performed over the entire array, the returned array must be a zero-dimensional array containing the test result; otherwise, the returned array must be a non-zero-dimensional array containing the test results. The returned array must have a data type of `bool`.
+
+
+(function-result_type)=
+### result_type(*arrays_and_dtypes)
+
+Returns the dtype that results from applying the type promotion rules
+(see {ref}`type-promotion`) to the arguments.
+
+```{note}
+If mixed dtypes (e.g. integer and floating-point) are used, the output of
+`result_type` will be implementation-specific.
+```
+
+#### Parameters
+
+-   **arrays_and_dtypes**: _List\[Union\[<array&gt, <dtype\]\];_
+
+    -   input arrays and dtypes.
+
+#### Returns
+
+-   **out**: _<dtype>_
+
+    -   the dtype resulting from an operation involving the input arrays and dtypes.