Add Documentation (#13811)

* Add more Docs and runnableExamples

lib/pure/concurrency/cpuinfo.nim

@@ -49,6 +49,9 @@ when defined(haiku):
 proc countProcessors*(): int {.rtl, extern: "ncpi$1".} =
   ## returns the number of the processors/cores the machine has.
   ## Returns 0 if it cannot be detected.
+  ##
+  ## .. code-block:: nim
+  ##   block: doAssert countProcessors() >= 0
   when defined(windows):
     type
       SYSTEM_INFO {.final, pure.} = object