provide better error messages for large set (#20207)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

doc/sets_fragment.txt

@@ -13,9 +13,13 @@ range `0 .. MaxSetElements-1` where `MaxSetElements` is currently always
 The reason is that sets are implemented as high performance bit vectors.
 Attempting to declare a set with a larger type will result in an error:
 
-  ```nim
-  var s: set[int64] # Error: set is too large
-  ```
+```nim
+
+  var s: set[int64] # Error: set is too large; use `std/sets` for ordinal types
+                    # with more than 2^16 elements
+
+```
+
 
 **Note:** Nim also offers `hash sets <sets.html>`_ (which you need to import
 with `import sets`), which have no such restrictions.