change os.nim doc links to new style (#19102)

doc/docgen.rst

@@ -399,6 +399,13 @@ recognized fine::
       no backticks: `func [][T](x: openArray[T]): T`_
       escaped:  `func \`[]\`[T](x: openArray[T]): T`_
 
+.. Note:: Types that defined as `enum`, or `object`, or `tuple` can also be
+   referenced with those names directly (instead of `type`)::
+
+     type CopyFlag = enum
+       ...
+     ## Ref. `CopyFlag enum`_
+
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