Update a comment describing the page queues.

Approved by:	re (hrs)

sys/vm/vm_page.h

@@ -224,18 +224,22 @@ extern struct mtx vm_page_queue_free_mtx;
 #include <vm/vm_param.h>
 
 /*
- * Each pageable resident page falls into one of four lists:
+ * Each pageable resident page falls into one of five lists:
  *
  *	free
  *		Available for allocation now.
  *
- * The following are all LRU sorted:
- *
  *	cache
  *		Almost available for allocation. Still in an
  *		object, but clean and immediately freeable at
  *		non-interrupt times.
  *
+ *	hold
+ *		Will become free after a pending I/O operation
+ *		completes.
+ *
+ * The following lists are LRU sorted:
+ *
  *	inactive
  *		Low activity, candidates for reclamation.
  *		This is the list of pages that should be
@@ -245,9 +249,6 @@ extern struct mtx vm_page_queue_free_mtx;
  *		Pages that are "active" i.e. they have been
  *		recently referenced.
  *
- *	zero
- *		Pages that are really free and have been pre-zeroed
- *
  */
 
 extern int vm_page_zero_count;