IntRange: Document lack of support for negative numbers

Fixes #895

source/int-range.d.ts

@@ -8,7 +8,7 @@ The numbers are created from the given `Start` (inclusive) parameter to the give
 
 You skip over numbers using the `Step` parameter (defaults to `1`). For example, `IntRange<0, 10, 2>` will create a union of `0 | 2 | 4 | 6 | 8`.
 
-Note: `Start` or `End` must smaller than `1000`.
+Note: `Start` or `End` must be non-negative and smaller than `1000`.
 
 Use-cases:
 1. This can be used to define a set of valid input/output values. for example:

test-d/int-range.ts

@@ -16,3 +16,10 @@ expectType<10>(stepTest2);
 
 declare const maxNumberTest: IntRange<0, 999>;
 expectAssignable<number>(maxNumberTest);
+
+// Not yet supported.
+// declare const negative: IntRange<-1, 1>;
+// expectType<-1 | 0 | 1>(negative);
+
+// declare const negative2: IntRange<1, -1>;
+// expectType<-1 | 0 | 1>(negative2);