Creating and setting comment nodes in macros (#5850)

* strVal assingnable comment statement ast nodes
* Set comment instead of strVal for comment nodes
* Added test code for creating and setting comment nodes
* Modified the AST spec documentation for documentation comments

compiler/vm.nim

@@ -1406,6 +1406,8 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
       if dest.kind in {nkStrLit..nkTripleStrLit} and
          regs[rb].kind in {rkNode}:
         dest.strVal = regs[rb].node.strVal
+      elif dest.kind == nkCommentStmt:
+        dest.comment = regs[rb].node.strVal
       else:
         stackTrace(c, tos, pc, errFieldXNotFound, "strVal")
     of opcNNewNimNode: