runtime(doc): Add missing null_<type> help tags (#13498)

Problem:  Not all null_<type> values have a help tag
Solution: Add missing help tags

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/vim9.txt b/runtime/doc/vim9.txt
index bba68ad..d10eae9 100644
--- a/runtime/doc/vim9.txt
+++ b/runtime/doc/vim9.txt
@@ -1025,8 +1025,9 @@
 Simple types are Number, Float, Special and Bool.  For other types |string()|
 should be used.
 			*false* *true* *null* *null_blob* *null_channel*
-			*null_dict* *null_function* *null_job* *null_list*
-			*null_partial* *null_string* *E1034*
+			*null_class* *null_dict* *null_function* *null_job*
+			*null_list* *null_object* *null_partial* *null_string*
+			*E1034*
 In Vim9 script one can use the following predefined values: >
 	true
 	false