thread_sync.c: fix documentation for Queue#close

When Queue#closed? is true, using push/enq/<< will raise an
exception, and not return nil.

* thread_sync.c: fix documentation for Queue#close
  Thanks to John Anderson.  [ruby-core:81749] [Misc #13673]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59165 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_sync.c

@@ -730,7 +730,7 @@ queue_do_push(VALUE self, struct rb_queue *q, VALUE obj)
  *
  * - +close+ will be ignored.
  *
- * - calling enq/push/<< will return nil.
+ * - calling enq/push/<< will raise an exception.
  *
  * - when +empty?+ is false, calling deq/pop/shift will return an object
  *   from the queue as usual.