mention how to create a decreasing sequence for indexing

Author: avehtari

src/reference-manual/expressions.qmd

@@ -804,6 +804,13 @@ given in the following table.
  |      all      |  `a[:]`  |     `a[1]`, ..., `a[N]`     |
  |      all      |  `a[]`   |     `a[1]`, ..., `a[N]`     |
 
+The range indexing with `:` allows only increasing sequences. Indexing
+with a decereasing sequence can be made by creating an integer array in
+the following way:
+```stan
+  array[6] int ii = reverse(linspaced_int_array(6, 2, 7));
+```
+Then `a[ii]` evaluates to `a[7]`, ...,  `a[2]`.
 
 ### Multiple index semantics {-}