Clarify output dtype guidance for mean, var, and std (#260)

Author: kgryte

spec/API_specification/statistical_functions.md

@@ -63,7 +63,11 @@ Calculates the arithmetic mean of the input array `x`.
 
 -   **out**: _<array>_
 
-    -   if the arithmetic mean was computed over the entire array, a zero-dimensional array containing the arithmetic mean; otherwise, a non-zero-dimensional array containing the arithmetic means. The returned array must have be the default floating-point data type.
+    -   if the arithmetic mean was computed over the entire array, a zero-dimensional array containing the arithmetic mean; otherwise, a non-zero-dimensional array containing the arithmetic means. The returned array must have the same data type as `x`.
+
+        ```{note}
+        While this specification recommends that this function only accept input arrays having a floating-point data type, specification-compliant array libraries may choose to accept input arrays having an integer data type. While mixed data type promotion is implementation-defined, if the input array `x` has an integer data type, the returned array must have the default floating-point data type.
+        ```
 
 (function-min)=
 ### min(x, /, *, axis=None, keepdims=False)
@@ -142,7 +146,11 @@ Calculates the standard deviation of the input array `x`.
 
 -   **out**: _<array>_
 
-    -   if the standard deviation was computed over the entire array, a zero-dimensional array containing the standard deviation; otherwise, a non-zero-dimensional array containing the standard deviations. The returned array must have the default floating-point data type.
+    -   if the standard deviation was computed over the entire array, a zero-dimensional array containing the standard deviation; otherwise, a non-zero-dimensional array containing the standard deviations. The returned array must have the same data type as `x`.
+
+        ```{note}
+        While this specification recommends that this function only accept input arrays having a floating-point data type, specification-compliant array libraries may choose to accept input arrays having an integer data type. While mixed data type promotion is implementation-defined, if the input array `x` has an integer data type, the returned array must have the default floating-point data type.
+        ```
 
 (function-sum)=
 ### sum(x, /, *, axis=None, keepdims=False)
@@ -196,4 +204,8 @@ Calculates the variance of the input array `x`.
 
 -   **out**: _<array>_
 
-    -   if the variance was computed over the entire array, a zero-dimensional array containing the variance; otherwise, a non-zero-dimensional array containing the variances. The returned array must have the default floating-point data type.
+    -   if the variance was computed over the entire array, a zero-dimensional array containing the variance; otherwise, a non-zero-dimensional array containing the variances. The returned array must have the same data type as `x`.
+
+        ```{note}
+        While this specification recommends that this function only accept input arrays having a floating-point data type, specification-compliant array libraries may choose to accept input arrays having an integer data type. While mixed data type promotion is implementation-defined, if the input array `x` has an integer data type, the returned array must have the default floating-point data type.
+        ```