Add some missing documentation to Queue#{close|deq} [ci skip]

Patch by Lars Kanis. [Fix GH-1949]

*  Describe the impact of Queue#close to Queue#deq .

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64756 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

thread_sync.c

@@ -799,10 +799,12 @@ queue_do_push(VALUE self, struct rb_queue *q, VALUE obj)
  *
  * - +close+ will be ignored.
  *
- * - calling enq/push/<< will raise an exception.
+ * - calling enq/push/<< will raise a +ClosedQueueError+.
  *
  * - when +empty?+ is false, calling deq/pop/shift will return an object
  *   from the queue as usual.
+ * - when +empty?+ is true, deq(false) will not suspend the thread and will return nil.
+ *   deq(true) will raise a +ThreadError+.
  *
  * ClosedQueueError is inherited from StopIteration, so that you can break loop block.
  *