[llvm] [TableGen] Enhance !range bang operator (PR #66489)

[Nicolai Hähnle via llvm-commits]

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@@ -1831,11 +1831,13 @@ and non-0 as true.
     result. A logical OR can be performed if all the arguments are either
     0 or 1.
 
-``!range([``\ *a*\ ``,``] *b*\ ``)``
-    This operator produces half-open range sequence ``[a : b)`` as ``list<int>``.
-    *a* is ``0`` by default. ``!range(4)`` is equivalent to ``!range(0, 4)``.
-    The result is `[0, 1, 2, 3]`.
-    If *a* ``>=`` *b*, then the result is `[]<list<int>>`.
+``!range([``\ *start*\ ``,]`` *end*\ ``[, ``\ *step*\ ``])``
+    This operator produces half-open range sequence ``[start : end : step)`` as
+    ``list<int>``. *start* is ``0`` and *step* is ``1`` by default. *step* can
+    be negative and cannot be 0. If *start* ``<`` *end* and *step* is negative,
+    or *start* ``>`` *end* and *step* is positive, the result is an empty list
+    ``[]<list<int>>``. For example, ``!range(4)`` is equivalent to
+    ``!range(0, 4, 1)`` and the result is `[0, 1, 2, 3]`.
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nhaehnle wrote:

Add an example with two operands to make it clear that they are mapped to start and end.

https://github.com/llvm/llvm-project/pull/66489