Change return type for "abs_diff"

Change `abs_diff` so that its return type is the same as the type of
its parameters.  This makes the signature consistent with `abs`.

Closes internal issue 655.

adoc/chapters/programming_interface.adoc

@@ -24189,14 +24189,12 @@ _Constraints:_ Available only if all of the following conditions are met:
 
 _Returns:_ When the inputs are scalars, returns |x - y|.  Otherwise, returns
 |x[i] - y[i]| for each element of [code]#x# and [code]#y#.  The subtraction is
-done without modulo overflow.
-
-When [code]#GenInt1# is scalar, the return type is the unsigned equivalent of
-[code]#GenInt1# (when [code]#GenInt1# is signed).  When [code]#GenInt1# is
-[code]#marray# or [code]#vec#, the return type is [code]#marray# or [code]#vec#
-with the equivalent unsigned element type.  When [code]#GenInt1# is the
-[code]#+__swizzled_vec__+# type, the return type is the corresponding
-[code]#vec# with the equivalent unsigned element type.
+done without modulo overflow.  The behavior is undefined if the result cannot
+be represented by the return type.
+
+The return type is [code]#GenInt1# unless [code]#GenInt1# is the
+[code]#+__swizzled_vec__+# type, in which case the return type is the
+corresponding [code]#vec#.
 
 a@
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